We have a similar schedule for our lo, and yours is about the same age as mine.
However, we also have a cat nap in there. Now, I know that our lo's are going to be dropping it soon, but I think right now they need it. Ours occurs anywhere between 3 and 4:30, depending on if she bombed her second nap or not. If she takes a long nap, we usually get her up by 2:30. For the morning nap, we usually let her sleep no longer than 1.5 hours.
The other thing you might try is extending wake time a little. My lo is up to almost 2.5 hours before the afternoon nap, and 2 hours 15 mins before the am. Short naps are usually due to not enough wake time, or too much wake time. In this case I suspect it's not enough wake time.
We go by the clock for the most part. She starts her sleepy signs about an hour before she's ready to go down, so we can't really go by those anymore. If we put her in front of several toys and she fusses, we know it's time.
Hope this helps a little!
*HUGS*
Sonya =P