I went through this with my DD when she was about 5 months and it continued for 1-2 months and i tried EVERYTHING. She was sometimes still awake happily moving around out bed at 10pm.. basially we introduced a bedtime routine:

5:00 pm dinner and play

6 o- 6:30 pm bath

7 ish - read her a book, bottle and bed

first few nights she fussed a bit but i sat in her room where she could see me till she fell asleep.It has been great ever since the week after we started the routine.

In the day i wait for tired signs but in the evening i do the same thing and she is always ready for sleep around 7-7:30 (depends when her last was)

Best of luck, it gets better