can baby's sleep cues change?!

For the first 2.5 months it was pretty easy to read my LO sleep cues- shed start yawning after about 1 HR of wake time. But lately she never yawns lol (she's now 3.5 months)- I'm still putting her down after 1.5 hrs or so but she has a hard time falling and staying asleep?! Should I be waiting longer for her yawn?! Maybe I'm putting her down too early?! Advice please!