We just went through this. The puzzling thing is that all of the sudden, they stop taking the pm nap, though they are exhausted. Lilah started to do this around 11 months, and it was not fun. about 2 weeks ago, we permanently moved to 1 nap. But in the interim, here's what worked for us:
Put her down at her usual time for a morning nap, but don't let her nap longer that an hour. I finally figured out that what was happening was that Lilah suddenly only needed about 2.5 to 3 hours of naps per day. She still was so ready for that nap in the morning, but if I let her sleep her usual 1.5 to 2 hours in the morning, she would not be tired enough for the pm nap. Even if I pushed it back all the way to 3:00! But if I woke her after no longer than an hour, she would almost always nap for me in the afternoon. Finally, she decided to skip the am nap. No problem! That one you can always make up for later. You want to keep the pm nap because eventually it will be the only nap left.
I hated to wake her after only an hour, but it really did work for us.