Potty Training *Warning* I'm talking about elimination here-pee & poop

I was so excited the first time Sophie used the potty, she was 21 months old and we were ahead of the curve! We have had the potty since she was about 18 months. It was easy to tell when she was having a "movement" and the next logical step to us was get a potty, put her on it, stop having her crouch in the poop corner.

She pushed her potty around the house, put toys in it, sat on it like a chair, returned it to its place of honor in the bathroom. The first time she actually used it she was mid-poop when I went to change her diaper so I scooped her up, set her on the potty and she finished her business. We both smiled and clapped! Put the potty in the big people's potty, flushed it away, washed our hands and commenced to mark this historic occasion with a butterfly sticker on the potty calendar.

As you can see there were only a few successes in January but they were successes nonetheless. We rolled into February.

Not many more attempts and successes than January but still she earned some stickers. We traveled early in the month and brought the potty with but then were hit by the Blizzard of 2010 so it's not surprising her "trying" got off kilter.
And then March came:

Spring has sprung! Look at all of those butterfly stickers! She was a potty champ. "You'll use the poooootttttyyyyy." Just like Elmo sings in "Elmo's Pottytime" video. We were on our way. I bought less diapers, bought pull ups, encouraged her to use the potty. We traveled again in March, toted the potty with, she used it all week. Even though she became constipated. She peed in the potty. I was a proud mama knowing I was potty training before age 2!

And then came April.

Nothing. Nada. No Dice. Zero. Zip.

She sits on the potty with CLOTHES on. Stands up. Claps. Congratulates herself with a hearty "Yea!" but has accomplished nothing. She loves her diapers. Hates the pull ups. I have no idea what gives or what to do. We use the potty together but she has nothing to show for it. I only have frustration.

Any suggestions? What has worked for your toddler?