I've only used yeast nutrients when making starters, where I add it in to the boil when I'm making my starter wort. Not sure if it is meant to be added to an already-fermenting beer, but don't take that as the final word by any means.
Personally, I think the best way to approach the issue is avoid stuck fermentations in the first place. Aerate well, pitch BIG (aka, make a starter), and keep your ferm temps where they are supposed to be.
Sorry, I know that doesn't help if you are dealing with a stuck ferm currently - if that's the case, have you tried gently swirling the fermentor to rouse the yeast, or bumping up the ferm temp a little? In a worst case scenario, I'd probably add a little dry yeast (US-56 or similar) before I'd add yeast nutrients to my wort...
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