My husband, who fixes machines, offers this advice: when setting the timing, always check final adjustment by turning flywheel forward to ensure that the timing and needle/hook clearance is correct. Do this before tightening the screws. Check again after tightening - sometimes a screw that is not flat can change the location of your hook. Also, tighten your screws as you would lugnuts on your tires, alternating between screws as you tighten them evenly. The second thing is to check needle/hook clearance. On many longarms there should be no space between needle and hook at rest. Too much space will prevent the needle from catching the thread when pulling the machine toward you. On my Crown Jewel and some others, rotating the needle between 5 and 7 o'clock may help.
I hope this helps you. If you wish, you can PM me and we will try to walk you through this. Ask us how we know about timing! Jeanette and Jim