The 404 is a wonderful machine.
When you bring it back to the technician, bring along a two layer narrow strip of your canvas, that you have done test stitching on to show him the problem. In fact, leave the last jam under the needle. Make sure there is enough canvas length for him to test sew with.
If you are making an airplane cover, I can imagine you are working with large pieces and quite a lot of weight and drag against the needle. It must be difficult to not "push" the fabric past the needle. Make sure your work is well supported. The feed dogs should be able to feed the fabric, your hands are there just to guide it.
I would also imagine that working with canvas you may need to change the needle more frequently. Are you using a sharp tip needle, vs. universal or ball-point?