How many props have you gone through. I am asking this because i had a similar problem where the prop broke upon hitting anything ( ground, grass, trees....). I went through 2 props and thought i would have to invest in many more but the third prop was a charm. It has not broken yet even with more abuse than the first to combined.

I can't explain why but if i had to i would say the the plastic props that come with the plane are probably old and britle and i you buy a couple of new one from your local hobby shop they may be better.