sorry i'm answering only now, I have been on hubpages for a while.
If you are still having a problem maybe one of these suggestions will work.
I am not to clued up on buttonhole machines but maybe you can look at the quality of cotton that you are using. I find that if I work with cheap thread on my serger/overlocker the left straight stitch thread keeps on breaking. But it could also be that you are using the wrong needles or it could be that the machines timing is out.
If the first two suggestions does not sort out our problem then it is most probably the timing.
I know that if you use old thread on an embroidery machine the thread also tends to break.
I hope this helps.
Please let me know, either way.