We've had this problem in the past.. it comes and goes - I call it the "transition" as in - if she makes it through the "transistion" then it will be a long nap. I have found she makes it through about once a day - and the other times - she makes it 40 - 60 mins. My only saving grace is at night she is going 5-7 hours before she wakes up first - and then sometimes giving me another 2 - 3 hours after. So I know she can make it through the transition.. so I've stopped worrying about it.. BUT when I was concerned and working on it I read post after post.. this is a common problem. Fixes include using the swing to help them sleep longer for a few days to help 'train' them to sleep longer - letting them fuss for a few mins - making sure you are putting down drowsy - rocking as soon as they start to wake (give binky back if they take one) - basically - anything you do to get them asleep - retry it before they get all worked up.
but in short - normal