Pfaff 2048 I had a bobbin locking problem. I removed the fabric and all visible threads but when I rethreaded the needle is hitting the bobbin casing if I turn handwheel forward, no problem if I turn it backwards

increase the needle thead tension

nothing to do with bobbins
A lot to do with insufficient needle thread tension

Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Verify the thread path is correct.
Set the top tension to the midway point (may need to tweak this for your particular machine).

Remove the bobbin thread from the machine. Reinstall the bobbin in the bobbin case confirming the bobbin is turning the correct direction and the bobbin case is correctly threaded.

Pull the bobbin thread up to the top of the fabric and gently hold both thread tails in your left hand when slowly taking the first couple of stitches.

Be sure to use a brand new needle.

Remove the upper thread and bobbin. Reinstall the bobbin making sure it is turning the correct direction and threaded properly in the bobbin case.

RAISE the presser foot and rethread the top thread. Confirm the thread pathway is correct.

Most often, knots (thread nests) occur under the fabric because the machine is misthreaded or the thread is not seated completely in the tension disk.

Sewing Machine Thread Bunching Up Here Why

What causes Bird Nests Superior Threads

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bunching of thread under material is a needle tension problem
tighten it up
proper tension is when it requires a small amount of effort to pull the thread through the needle eye

Check that it is actually the bobbin thread that is nesting. Generally, bobbin thread issues appear on the top of the fabric and upper thread issues appear on the underside of the fabric.

If it is in fact the bobbin thread, the bobbin thread tension may be off or malfunctioning.

If it is the upper thread nesting under the fabric:
1. Remove the thread
2. Install a brand new needle
3. ALWAYS RAISE the presser foot. This releases the thread tension so the thread will seat properly.
4. Rethread from the beginning and check the thread path is correct.
5. Set the upper tension at the midway point.
6. Check that the bobbin thread is loaded properly
7. Retest the machine.

If it continues to create thread nests under the fabric, tighten the upper tension.

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Try these suggestions

1. Change the needle--use a needle that's for the fabric being sewed

2. Rethread the machine--top --make sure to use every thread guide

3. try a different bobbin--make sure bobbin is inserted correctly into the bobbin case, & make sure the bobbin is turned the correct way in the bobbin case

4. make sure the thread has not come off the thread take up lever

5. Clean under the needle plate & in the bobbin area for lint or threads

6. Also could be a timing problem, and the machine would need to be checked.

Try these suggestions
1. Change the needle--use a needle that's for the fabric being sewed
2. Rethread the machine--top --make sure to use every thread guide
3. try a different bobbin--make sure bobbin is inserted correctly into the bobbin case, & make sure the bobbin is turned the correct way in the bobbin case
4. make sure the thread has not come off the thread take up lever
5. Clean under the needle plate & in the bobbin area for lint or threads

try changing the needle
clean under the needle plate & in bobbin area for loose threads & lint
try rethreading the machine top & bobbin, make sure to get every thread guide
make sure the bobbin is inserted correctly in the bobbin case

Good that you changed the needle. Did you rethread just the needle or did you rethread the machine? You may have already taken these steps, but if not, here is what I would suggest:

1. Replace the old needle (which you did), even if it looks ok, as it may be bent or have a tiny burr or other damage that isn't easily visible. Make sure the needle is seated properly - facing forward and fully inserted.
2. **Completely rethread the machine, making sure the thread is going through the take-up lever. It can easily pop out when the machine jams, and you can't always see that the thread is out of place. That could explain why the thread doesn't loop properly around the bobbin.
3. Remove the bobbin and clean any knotted thread or lint from around the bobbin carrier. Canned air is great for cleaning the lint.
4. Replace the bobbin, making sure the thread-end is clipped neat, not frayed or knotted.
5. Try sewing on some practice fabric to make sure the stitches are going to form properly before returning to your sewing project.
Good luck!