Lexi's worrying patterns; flyball version

We've had a tough couple weeks the last two times we went to flyball.

Basically Lexi's turns at home are great.  I took her to class and even without the ball, and without the trigger, she just doesn't want to put 4 feet on the box.  I'd get 2, or 3 (with the third way down at the bottom).  I don't think she put 4 on a SINGLE time.  I had her out there with treats and she just wasn't doing it.  She was revved up and excited, I tried running with her, sending her, etc...nothing doing.

So, I decided to work on something else.  I started with triggering the box myself and throwing treats out.  Lexi did not appear worried about the noise at all, and was very excited about the sound since it would immediately lead to treats.  So, I started having her trigger the box and I would throw a ball--awesome, no trouble (she was just pushing with two feet, but I figured that was okay since I was just working the noise/feel).  Then I had her actually triggering the box to get the ball.  She did it about 3-5 times, was excited, up, happy, etc...and then all of a sudden, she wanted nothing to do with it.  She went from triggering the box and fetching the ball to refusing to put any feet on the box at all.

We took a long break, then I went back just a few more times and had her trigger the box for a cookie, and we called it a night.

This reminds me of what she did with the teeter.  No issues that we could see, but one day she would just refuse it.  We must have retrained it 3 times.  Looks like a similar pattern with the box.