Nap extensions

Tips? Tricks?

Daughter is 20 weeks old. Goes down for a nap every 2-2.25 hours in her crib. Puts herself to sleep and wakes up 45 minutes on the dot.

I've tried crib hour, going in to wake her a little before the 45 min to restart her sleep cycle, I've rocked her and she goes back to sleep but I can't put her back down to resume her nap. She just cries.