There is two kind of skill effects:

1- Skill effects: knockdown, knockup, knockback, bound, daze, freez
these group following up probability rules, so it may the skill effect not worked all the time and make a chance to scape from your attack by the opponent.

2- Skill effect: Grab
grab skill worked all the times, unless opponent use skill that cover "can not be grab"

most of classes has the skill that comment "can not be grab", but it is not make full resistance because of two reason:
1- Unsuperarmor skills that comment "can not be grab"
2- superarmor skill that comment "can not be grab" + "while strating" or "while jumping" or "while moving"

The suggestion:
It's better antigrab skill made on superarmor skill and comment: "can not be grab during whole time of the skill".

Thanks.