According to the Mahabharata, Balarama is mentioned to be an incarnation of Adi-Shesha-Naga:
And Baladeva of exceeding strength was a portion of the Naga, Shesha. (Mahabharata 1.61.91)
On the hand, the third chapter of the first canto of the Srimad Bhagavatam states Balarama to be an incarnation of Vishnu:
In the nineteenth and twentieth incarnations, the Lord advented Himself as Lord Balarama and Lord Krsna in the family of Vrsni [the Yadu dynasty], and by so doing He removed the burden of the world. (Srimad Bhagavatam 1.3.23)
The two "contradictory" verses are reconciled by the Srimad Bhagavatam itself in the second chapter of the tenth canto of the Srimad Bhagavatam:
When Kamsa had killed six of the children born from Devakî, the seventh one, a plenary expansion of Vishnu who was celebrated with the name Ananta, therefore [was born] as an embryo in the womb of Devakî ... [Vishnu] instructed His spiritual potency [Yoga-mâyâ] as follows: ... "In the womb of Devakî there is the embryo known as [Ananta-]S'esha who is a plenary expansion of Me. Take care of a smooth transition from her womb to the womb of Rohinî.... Because He changes wombs [from Devakī to the womb of Rohinī] the people of the world will address Him with the name Sankarshana, because He brings pleasure to the people [of Gokula] He will be called Râma and because of His great physical strength He will be named Balabhadra."
Thus, Balarama can be regarded aa an incarnation of Vishnu. Similarly, many Sri Vaishnavas who claim Ramanujacharya as an incarnation of Adi-Shesha-Naga at times, call Him an incarnation of Vishnu. So if Ramanujacharya and Balarama can be considered incarnations of Vishnu since Adi-Shesha-Naga is simply an expansion of Vishnu Himself, why is Lakshmana, who is also considered as an incarnation of Adi-Shesha-Naga, not called as an incarnation of Vishnu?