DD is 21 weeks old now, and we have always struggled with daytime naps. Realised how much after pub lunch out with other mums from our antenatal class, while all the other babies dozed happily in their prams 
DD grizzled throughout and we couldn't settle her.
We don't have problems with nighttime sleep, she'll do a 7 - 6 solid (on a good night...). We just can't get her to self-settle during the day. She gets tired, rubbing her eyes, yawning and getting grizzly, but we can only get her to sleep after 10mins+ of vigorous rocking, shushing etc. She puts up a fight! We go through this for all 3 daytime naps, about 45 mins each. Pram/ride in car works no probs, but how do we teach her to rest without needing to be rocked?
So... tell us what we are doing wrong so that we can have a snoozy baby too :) Or is this just how she is? I it started up as she was so colicy in the early days, she is just used to constant rocking to settle her.