I would suggest you do the usual...change the needle, check the thread path making sure all contact points have thread running through them correctly, check the stabilizer to be sure it's appropriate for the type of fabric and design you're stitching, and possibly change the speed of the machine. I have the 1501 machines, and they have a little pig tail on the bobbin case. Make sure this pigtail is present and isn't bent. I had one break off and it took forever for me to notice that! When I replaced it, though, the stitch quality returned.
Skipping stitches is typically caused when the hook fails to catch the loop in the top thread. That part is easy. Finding out why is not.
Assuming you haven't done anything to throw off the hook timing and also assuming it ran well on the previous job and assuming you're not getting birdsnests, then flagging of the material is a good place to start. If the material pulls up with the needle as the needle is retracting, the loop fails to form and the hook misses. Some machines have adjustable presser foot that can help wit this, sorry I'm not familiar with SWFs in particular.
And make sure that when hooped and on the machine, the material/backing are in contact with the needle plate.
You can very quickly determine if the problem is above the needle plate or below by moving the problem color to another needle as well. If the problem then goes away, look at needle and upward on the offending location. If the problem still exists look at the needle plate and downward.
It could be the design if the densities are way, way off, but it would have to be way off.
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