If she is still taking naps that may be the culprit, especially naps over 2hrs. If she takes naps during the day, she will fall asleep later, there is no way around that. My son stopped before 2yrs old. He wakes up at 8-830 up all day then in bed by 7pm. Like clockwork.
Now if she is just in bed and not being fussy but just awake, let her be. Sometimes your presence in the room is distracting her. If it were teething or some illness you would definitely know, she would be fussy or crying or something to tell you she is uncomfortable.
For my niece music helped her, she had a little glow worm that played when you squeeze it. Granted it might keep you awake, lol, but she loved it and it helped her fall asleep quicker.
Also you could try reading to her. I read to my son every night since he was born. Its a great routine and it helps him settle down before bedtime.
Hope this helps you.