Patchy heat from new vertical radiator

Due to storage, nearly all my rads are now vertical with no problems except one bedroom. It heats one panel of four, but heats up properly if whole system on full. My son's autistic and says his computer heats his room, but he's compared other bedroom rad using his thermal camera and the one in his room seems very "lazy". It was lazy before I turned flow rate down to save energy, but turning it back up, bleeding the rad, hasn't made any difference.