Gohan may have had potential, but he was useless without other characters pulling that potential out of him. He was legit trained by Piccolo, then on Namek he was just handed a power up by Grand Elder Guru. He was later trained by Goku who helped him achieve Super Saiyan and eventually Super Saiyan 2, but with Goku out of the picture and no enemies to fight he slacked off and got significantly weaker. That is until he had yet another power up handed to him, this time by Old Kai. So yeah... I think it makes total sense that Future Gohan never surpassed Super Saiyan given how reliant he was on others to increase his power.
Future gohan never slacked off though and was always constantly fighting the androids and gettin his a$$ kicked so he shoulda recieved a power boost every time he fought for like the past 15 years of fighting the androids