is you bobbin thread catching with the top threads. No reason for the seams to come undone. Do you backstitch? If you don't do you perhaps clip the threads too short before attaching to the other rows/pieces?
If not, then I have to agree with the others
Sewing at 1/2 inch isn't going to be any better then at 1/4. If it's coming apart it will come apart, no matter how big your seam