LRM, it could be by the time he yawns he's OT, or in the annoying language of those busy-body-smug books you have "missed the window"! If it happens pretty regularly, maybe try putting him down ten mins before signals. You may have noticed some pre-yawn behaviour?
It might even be worth introducing a little pre nap 'routine' like a short book, a song and a little phrase. That way if it works he will later associate that with napping even if he is OT.
If that has been tried/doesn't work, some other OT ideas include white noise (I have not used but am v impressed by results; many apps available), or reassurance techniques like shush-pat etc. (that one Baby Whisperer I think, certainly Bw blog site will have lots of non-cry-it-out suggestions).
It might be worth dealing with one issue at a time. ie. if you now rock him to sleep, start by getting him to fall asleep not being rocked (even in crib) but with you there, just gently reassuring him with voice in the room, or with hand on tummy. When that is working withdraw hand. Then gradually withdraw from the room. Instinct etc will tell you the right times/etc.
Finally my view would be devise a plan that works for you and baby monkey, then stick to it for a few days, at least. Consistency is important. And you might feel (and radiate) calmer if you have committed to a system for a while. Also I used to give a time limit to myself in advance, for similar reasons, but don't then rock to sleep - do something else like sing a couple or rhymes then go for a buggy walk - even if he naps in buggy you have not rocked him to sleep
Forgive me if all too obvious or old hat. I am a veteran of napping, er, issues, but you might be a lot more experienced or talented at this than I was 
Hope it goes well
Sx