Theeding the needle

This can be a tricky adjustment, Make sure you have the correct needles inserted correctly. The hookwire isn't lined up properly with the needle bars holding the needles. Expensive machine to break trying to fix it. Have a trained professional Take a look at the threader mechanism, you will have the opportunity to learn something about your machine in the process.
Cheers,
Embroidery Services
San Diego, CA

look down in there there is a hook to grab the top thread may have gotten bent or loosend , you may also try comparing the new neddle with the old and be sure it is correct, look for a small latch under the grab foot, please rate -jeff

Manually feed the bobbin thread through the needle top plate and try sewing. The result might give you a further clue as to what's going on. If its not something as obvious as excessive tension or incorrect routing of the bobbin thread, it might be the clearance between the needle and the needle hook. Did you try changing needles in case it's a bent needle?

You probably haven't inserted the needle fully into the needle bar. Try that first.

Also check that there isn't a bit of needle shank in the holder.

Good luck.

Skipped stitches
1. The needle is not inserted correctly
Remove needle & reinsert needle( flat side towards the back of machine)
2. The needle is damaged
replace with new needle
3. The wrong size needle is being used
Choose a needle to suit the thread & fabric
4. The foot not attached correctly
Check & attach correctly

if you can send me the model number of the machine,, I'll see if
singer has a free downloadable manual

The needle thread breaks
1. the needle thread is not threaded properly
2. The needle thread tension is to tight
3. The needle is bent or blunt
4. The needle is incorrectly inserted
5. The needle thread & the bobbin thread are not set under the presser foot when starting to sew
6. the threads were not drawn to the rear after sewing
7. The thread is either to heavy or to fine for the needle

You may have knocked the machine out of time if the needle is hitting somewhere, email me and I will attach timing instructions, [email protected]

the bobbin should fit under the steel plate you'll have to make the needle stay up to remove the base plate

pfaff expression 2.0 works the same. I spend lots of hours with the thread tension and it don;t give me a good stitch. the needle threed ends up making large irregular loops .

I assume you checked to see there are no needle bits in the bobbin case holder.
Without looking at the machine it is difficult to diagnose the problem. It could be any one or a combination of problems, timing, needle bar height, damage to the hook and base etc.
I strongly suggest you have it looked at by a sewing machine tech.