DD (just turned 3) sleeps around 7.30pm until around 6.30am. She has a 2-2.5 hr nap starting between noon-1pm (depends when her little sister goes down - I put them down at the same time).
OP - sounds like you've tried lots of things. The only things I can suggest are consistently putting her to bed earlier, looking at her diet to see if this is triggering lack of sleepiness, and does she have toys etc in bed? DD is allowed to choose one toy for bed and if she mucks around that gets taken away (never happens - she just rolls over and goes to sleep). Make bedtime as boring and non-stimulating as possible!