I don’t think he can just automatically do it, I think the other person has to be weakened enough for his current level to overpower them (?) honestly I don’t know either lol but that’s what I’ve been assuming considering he wasn’t able to do it with the S-Rank guy and now it seems he’s having issues with it in regards to the brothers
from what I've gathered, I just assumed so far that predation can only be activated if his fatigue isn't too high, the opponent uses a skill that's new to them, and he has to have enough mana to activate it. and he won't 100%-? maybe not get the skill that was used since the flame guy used an entirely different skill he gained or did not get a skill at all. He's done a bunch of dungeons and fought a bunch of monsters who might have skills like those spider sisters but didn't get a skill and the reason it didn't work on the assassins is because they have the same skill stealth that the mc already has. He also can't improve his rank or "level" (to make it simpler) easily because he has to fully absorb something stronger than him to do so OR he has to do it on numerous occasions similar to exp being gained in games. I still don't get it but that's the best I can do on assumptions. Hope it helped

I still don't understand how predation works. I thought I've gotten the gist of it