I know how frustrating this is! We just transitioned fully to one nap. What really helped us was to keep the morning nap very short (about 45 minutes, absolutely no longer than an hour), then don't put your dd down again for at least 3 hours, 3.5 if she can make it. Make sure her tummy's full. Let her nap as long as she will sleep for that second nap. Our original problem was that Lilah was dropping the afternoon nap after one longish (1.5 to 2 hour) morning nap, then staying awake from 11:30 or so until 7:30 at night! So not fun for any of us. When we shortened the morning nap, she slept well for the morning cat nap, then usually slept well again for the afternoon nap. Then one day she refused the moring nap. So much easier to drop that one; I just put her down earlier for the afternoon nap. We had a couple weeks of sometimes one nap, sometimes two (I just followed her cues), but now are on one nap. It is usually 2-3 hours, but sometimes we get a short one (only one hour--not good). It's much better than that transition time, though. She goes down right after lunch, between 12:00 and 12:30.
HTH!