Brother XL 3520 not picking up bobbin thread on zig zag?

Hi, I have just aquired a Brother XL 3520 and the straight stitch is fine, however if I try to zig zag or any other built in stitch the right had stitch is missing...looks like it is not picking up the bobbin thread on right hand side.The machine had no standard presser foot when I aquired it (which I have now ordered ).I am stitching with the zipper foot at the moment so wondered if this could be the problem ? I have tried changing the needle and cleaned the bobbin case.
Could it be timing ? or is it simply the wrong foot that is the problem ?
Thankyou in advance.