I doubt they are getting out. They may be bumping the loop and/or other animals may be bumping them. Most successful snaremen set ALOT of snares. Make up a mess of them and cover the trails with them. In many states it is illegal to use a "spring pole" set with a sapling and in some states you can't even snare or trap some "game" animals, including small game animals. Not saying its a personal issue--survival and self sufficency need practiced not just read about--but be aware of whether you are within the bounds of the laws of your area.

Make sure your snare is closing with practically no resistance. Use grasses, twigs, etc. to fence the trail down to the edge of the snare loop. Also be aware that anything is liable to end up in the snare, not just rabbits. Even a larger animal can get leg snared. Make sure you are checking them at least daily.