As others have said, it is likely your axle is a smidge too long. Cut 2 or 3 mm off with a hacksaw and smooth the end with a file and you will be good to go.

If it still slips when u r crankin then maybe you are not gettin enuff force from yur QR. Try a bit of oil in the pivot of the QR or, if it is a newer open-style qr, then put a dab of grease on the concave nest where the cam part of the lever goes.

GOod luck!