It cuold be any number of things. My only real advice is to try not to try so hard
If she won't sleep, just let her be awake. I know she needs to. I know she's knackered and overtired, but it really isn't worth the effort. I gave up on day sleeps with DS from around 10 months or so. By 16 months he was into a fairly predicatble routine - just happened eventually. There are few things worse than spending an hour (or two) to get them to sleep for only 30 minutes.
The only time I really kept trying was at bed time - it was better when it was only once a day.

Hang in there. It will get easier