Whatever is making Skeletor reset on death overrides this soul seal spell, and as is the case in most stories interrupted or failed spells come at a cost.
It actually makes sense that whatever power makes him reset is way more powerful a magic than the sorcerer is capable of and therefore can't be overwritten by a soul bound spell.
MC is only surviving due to stupid-ass plot armor. I would prefer if he wasn't such a massive bitch and instead fought against the control, instead of this stupid "author" interfering with shitty plot.