S.D.
I had trouble with both my boys around 4-6 months. I found three things: 1) I was missing sleep cues and they were PAST needing a nap; 2) they needed black-out shades so the room was dark; 3) "The Baby Whisperer" had compassionate alternatives to CIO that said respond every time baby cries. It takes some work, but you'll get back to good naps:)