First off it appears in your pictures you are having stitch formation problems with the upper/top thread. I do not know if you have a home or commercial machine as exact methods to fix the problem will differ but basic problem/solution should be similar.
Be sure to only try one thing at a time when attempting to fix your problem!
First - Completely Re-Thread your machine from scratch and insure that your bobbin rotates in the proper direction during operation. Having a reversed bobbin will cause a multitude of problems as will missing just one thing in your threading. (Hint - I've done this myself - check your upper tension disks, be sure your thread is between the disks and not behind them! It looks right at a glance but can result in your problem)
2) Check that your tensions are correct, both upper and bobbin. On your bobbin make sure there is no lint build up under the bobbin case tension spring, with no bobbin tension you will get the results you show.
3) Check that your needle is installed correctly, that it is the proper size and doesn't have any burrs or a bad point.
4) Be sure that your item is hooped properly and is not flagging as you are sewing.
5) Slow your machine down to lowest speed and increase speed slowly if it sews well at slow speed. Watch it sew and look/listen as it sews for any off noises as you slowly speed up the machine.
I would say that your problem in 90% + a tension problem of some sort, either by not enough tension on top, too much bobbin tension, thick material/backing, possibly timing issues. Start with the simple then go from there.
Hope this gets you on track.
Why is this happening??
