I think lack of professionalism from the start has contributed a big part of it - he was commonly cited for poor dietary, sleep and rest & recovery habits. He has since improved in most of those areas but the reality is that more than a third of hamstring injuries are recurring - they simply take time to heal & strengthen, if one adheres responsibly to the recovery program. That, is anyone's guess. From there, either the physios & trainers are missing a trick at the club or the kid is potentially injury prone. Problem is he relies on his explosiveness and the injuries are occurring in the same area of his hamstring
In his defense, he has taken it hard as he went off in tears after his 9th muscular injury at Barca
Valverde pointed out in his 1st season at Barca that the player hadn't experienced muscular problems before and at that time suggested it was down to inexperience recognizing the signs as a professional footballer or even succumbing to the pressure of his big move from Dortmund. Players are supportive of him, Messi has been effusive in his praise of the player but everyone is frustrated. Perhaps it will improve with a change of scenery to a new club but I do not believe his near future will see him remain at Barca