It has to be understood that game of dice was played twice. First time, Pandavas lost their kingdom and themselves too (became Daasa/slaves). Draupadi never became daasi in the opinion of wise like Vidura because she was staked after Yudhisthira himself lost. Later, even Arjuna confirmed that Yudhisthira lost himself first so he had no right to stake Draupadi. Due to insult shown to Draupadi as a daasi (which she was not) many bad omens happened there and terrified by the bad omens, Dhritrastra asked Draupadi to demand boons:
- In first boon, Draupadi asked freedom of Yudhisthira.
Dhritrastra granted the boon and asked Draupadi to wish another boon.
- In second boon, Draupadi asked freedom of Bhima, Arjuna, Nakula and Sahdeva
Dhritrastra granted the boon and asked Draupadi to wish another boon. But Draupadi said that Kashtriya women can't ask more than 2 boons and said that her husbands will again earn wealth as they are free now.
But, terrified Dhritrastra returned the wealth won in dice to Yudhisthira:
Dhritarashtra replied.--'O Ajatasatru, blest be thou. Go thou in peace and safety. Commanded by me, go, rule thy own kingdom with thy wealth. ...
~Sabha Parva: SECTION LXXII
The same was confirmed by Shakuni:
Sakuni then said,--'The old king hath given ye back all your wealth. ...
~ Sabha Parva: SECTION LXXV
This was over and Pandavas left for their kingdom i.e. Indraprastha. But now, Duryodhan requested his father to call Pandavas one more time. He said they will not forgive the insult done to Draupadi, so let there be one more game and either they will go in woods for 12+1 years or we will go.
And, Dhritrastra sent a messenger to call back Pandavas (who had by that time gone far to Indraprastha). And commanded by their uncle, Pandavas came back to play for which the condition was:
Sakuni then said,--'... Either defeated by ye at dice, dressed in deer skins we shall enter the great forest and live there for twelve years passing the whole of the thirteenth year in some inhabited region, unrecognised, and if recognised return to an exile of another twelve years; or vanquished by us, dressed in deer skins ye shall, with Krishna, live for twelve years in the woods passing the whole of the thirteenth year unrecognised, in some inhabited region. If recognised, an exile of another twelve years is to be the consequence. On the expiry of the thirteenth year, each is to have his kingdom surrendered by the other. O Yudhishthira, with this resolution, play with us, O Bharata, casting the dice.'
~ Sabha Parva: SECTION LXXV
Now, again Yudhisthira lost and as agreed spent 12+1 years in the woods which is described in Did Pandavas really complete their ajnatavasa? post.
As per the condition, on the completion of 13 years, Yudhisthira was supposed to get his kingdom back. But, Duryodhana didn't return the kingdom which clearly mean that Duryodhana is doing Adharma.
Now, it should be clear that Pandavas completed the 12+1 years exile and should get their kingdom back as per the conditions. So, Krishna favoured the Pandavas!
Lord Krishna listed lot of adharma of Kaurvas in this chapter of Mahabharata: Udyoga Parva:
[Lord Krishna to Duryodhana:] Thou thinkest, O thou of little understanding, that thou hast committed no offence against the Pandavas? Let the (assembled) monarchs judge. Grieved at the prosperity of the high-souled Pandavas, thou conspirest, O Bharata, with Suvala's son about the gambling match. O sire, how could those virtuous, honest, and superior kinsmen of thine (otherwise) engage in such a wicked act with the deceitful Sakuni? O thou that art endued with great wisdom, gambling robs even the good of their understanding, and as regards the wicked, disunion and dire consequence spring from it. It was thou who hadst devised with thy wicked counsellors, that terrible source of calamity in the form of the gambling match, without consulting with persons of righteous behaviour. Who else is there, capable of insulting a brother's wife in the way thou didst or of dragging her into the assembly and addressing her in language thou hadst used towards Draupadi? Of noble parentage, and endued with excellent behaviour, and dearer to them than their very lives, the queen-consort of Pandu's sons was treated even thus by thee. All the Kauravas know what words were addressed in their assembly by Dussasana unto those chastisers of foes,--the sons of Kunti,--when they were about to set out for the woods. Who is there capable of behaving so wretchedly towards his own honest kinsmen, that are ever engaged in the practice of virtue, that are untainted by avarice, and that are always correct in their behaviour? Language such as becomes only those that are heartless and despicable, was frequently repeated by Karna and Dussasana and also by thee. Thou hadst taken great pains to burn to death, at Varanavata, the sons of Pandu with their mother, while they were children, although that effort of thine was not crowned with success. After this, the Pandavas with their mother were obliged to live for a long while, concealed in the town of Ekachakra in the abode of a Brahmana. With poison, with snakes and cords, thou hadst, by every means, sought the destruction of the Pandavas, although none of thy designs was successful. With such feelings when thou hadst always acted towards them so deceitfully, how canst thou say that thou hast not offended against the high-souled Pandavas? Thou art not, O sinful man, willing to give them their paternal share in the kingdom, although they are begging it of thee. Thou shalt have to give it them, this, when divested of prosperity, thou shalt be laid low. Having, like a heartless fellow, done innumerable wrongs to the Pandavas and behaved so deceitfully towards them, thou seekest now to appear in a different garb.