this sounds exactly like our DD. up until about 4 days ago, her bedtime was 9:30pm-10:30pm. one day i just upped her last feeding of the night by half an hour, and then the next night, did it again, and for the last 3 nights she was out by 8:30pm and all three times she slept her normal amount of hours, until last night, she actually slept an extra 2 hours!!! and it seems like she sleeps much easier for her naps during the day now!
just little changes over the course of a few days should help, it did for us!