Greetings! Been enjoying the game quite a bit and I've got curious why the grapple doesn't work sometimes. There are enemies that can't be grappled - like the big ogre lady with a club, but ones like harpy should always be able to be put into full nelson. Although, even when harpy doesn't fly, the grapple option seems to be missing. And so happened on several enemies that are supposed to be grappleable! Like wolfgirl or goblin, the grapple option just disappears. It doesn't seem to be status effect or battle modifier not letting me grapple them, nor their stance. No idea!

Big orge lady have very very high strength. You have to weaken her or make yourself stronger (by magic and potions). 

Can't say much about not able to grapple, haven't seen situation where was no reason to not be able to. You can run out if stamina, then you have to hold in position while your stamina regenerating (thats endurance dependant). When you're injured you have less endurance, hence faster you loose your stamina.

It's hard to tell if harpy flies or not in opinion of the game. After some time in fight eventually she becomes grappable. And imagine grappling an ogre in real life.