Doing free motion quilting, feed dogs down, and the thread keeps jamming under the plate. It jams the needle and there's several small lengths of thread maybe 1/2 inch jammed below.

Can I Free Motion Quilt on a Regular Sewing Machine? By National Quilters Circle

Yes

https://www.nationalquilterscircle.com/article/can-i-free-motion-quilt-on-a-regular-sewing-machine/

How to Stitch the Perfect Stitch Length for Quilting
JULY 28, 2020
Today’s Topic: How to Stitch the Perfect Stitch Length for Quilting

https://lorikennedyquilts.com/how-to-stitch-the-perfect-stitch-length-for-quilting/

Look for a lever under the plate that disengages the feed dogs for free form quilting. More detail would help. My wife is a quilter.

Could be any of a multitude of reasons.

Check whether the feed dogs are actually rising 'above' the needle plate. If not, then they may be in a dropped position used for darning and free motion quilting.

Sometimes the feed dogs and the mechanism underneath is jammed with lint and debris. Make sure the dog teeth are well cleaned. Also check if the tips of the teeth are worn. Also verify that the needle plate is flush with the free arm--if it is raised, it will prevent the feed dogs from doing their job.

The other thing to check is whether the machine has a presser foot pressure adjustment. Some machines have it, others do not. On newer machines, it is usually a dial somewhere around the left side of the head or toward to top of the head. On older machines, unfortunately, some are simply a big screw head on the inside of the case directly above the presser foot bar. It basically applies more pressure to the presser foot, which in turn applies more pressure to the feed dogs. The problem will usually show up when sewing either very heavy or very fine fabric. Perhaps trying a different presser foot will make a difference.

I don't know what is the highest number the stitch length goes to on your machine, but a #2 stitch length is pretty short on mine. Try setting it higher.

Sewing Machine Presser Foot Stuck

Singer 6235 User Manual

For feeding the material, check that the feed dogs are in the "up" position and not set for darning or free motion quilting.

For the thread jam:
Clean the bobbin area of any lint, threads, or debris.
Be sure to clean the feed dogs and under the needleplate.
Install a brand new needle.
Remove the thread from the machine.
ALWAYS RAISE the presser foot when threading.
Rethread the machine from the beginning.
Verify the thread path is correct.
Set the upper tension to the midway point
Retest the machine.

...

Ashleigh--Your machine will work satisfactorily ONLY if the presser foot is down. When the presser foot is up, the tension disk is released, therefore, no tension = a big knot of thread.

To free motion quilt, you should drop the feed dogs. Install a darning foot (this foot is shorter in length so when it is lowered, there is a space between the presser foot and needle plate). The space allows the fabric to be moved easily while stitching.

Some sewists will free motion quilt without a presser foot (if they don't have a darning foot), but that is very dangerous as one can easily sew through a finger. However, even when sewing without a presser foot, the presser foot mechanism needs to be lowered in order to properly form stitches.

How to Free Motion Quilt on Regular Sewing Machine

1. The top loading bobbin cases have small brackets to keep them from turning or moving up or down.
2. Adjust brackets with small screwdriver. Allow enough room for thread to flow through.
3. Rotate hand wheel both directions to make sure the bobbin case doesn't move around. Also, watch the thread to see if it is flowing across the top of the bobbin case.
4. You may need to slow down movement of your "free motion". Also, make sure you are using the right pressure foot.

What make and model sewing machine?

I am guessing here because I don't know what kind of thread you are using but, I am guessing that you have something in the upper tension discs causing the thread to not have tension, first I would recommend if you can see in to the disc put the foot in the up position and and blow out the disc. If this doesn't work take it to you local dealer and have the tensions adjusted, also look at the needle plate and see if there are marks in the hole, if there are it would be wise to either clean them out or get a new needle plate they will severely affect free motion

this is a known issue with the pfaff.....i have just bought the machine and it has thread uptake issues...thread tension issues and it comes unthreaded by itself often...leaving globs of thread on the bottom stuck in the feed dogs and under the throat plate.....then the stitches are not properly balanced because the tension gets too loose on the bobbin thread....very disappointed to find out it is a known issue with all the viking husqvarna made machines...which are pfaff, janome and viking.....sux.

I had the same dissatisfaction when I switched from the 1+ to the Designer 1. After some trials I am now happy. You need the springy free motion foot. Part #413 03 76-45. You take off the white shank and screw it on with the metal bar over the needle hook. In the set menu turn the SensorQ on, the free motion on, select stitch 1 from menu V and drop the feed dogs. It also makes a big difference to use the straight stitch throat plate which just has a small hole for the needle to go through. One other hint is that I often have to reduce my needle tension 3 or 4 steps. Hope it works for you.