If you've tried everything, your timing could be off just a little. As a temporary fix, you can move the needle position to the left or right a bump or two. That will change the timing just a little until you can get it to the shop.
Of course, some of the more sophisticated machines may not let you do that. You may have to lie to it and tell it that you're doing a straight stitch with the standard presser foot. And you won't be able to use the straight stitch plate. And the needle won't be centered in the middle of your presser foot. But it might sew without skipping.
You'd know if the timing was off by sewing a regular zig-zag stitch on a scrap about 7" long. Sew the length at least 3 times. If it's skipping stitches, you know the timing is off.