A nest of loops under the fabric is almost always the top thread, not the bobbin. It happens when the top thread is not seated between the tension discs — usually because the machine was threaded with the presser foot down, which holds the discs closed. Unthread completely, raise the presser foot, rethread, and test.
Why it looks like a bobbin problem and is not
A stitch is made from two threads. The needle carries the top thread down through the fabric; the hook catches the loop and wraps the bobbin thread through it; then the take-up lever rises and pulls the slack back up so the two threads lock inside the fabric.
That last step is the one that fails. If the top thread has no tension on it, the take-up lever cannot pull the slack back — so all the excess loop stays down there, underneath, and each stitch adds another one. What you are looking at is not bobbin thread that has gone wrong. It is top thread that never came back up.
Which is why adjusting the bobbin tension screw, which is where a lot of people go first, makes it worse. Leave the bobbin case screw alone.
The fix, in order
1. Rethread the top with the presser foot UP
This resolves the large majority of cases. The presser foot lever is mechanically linked to the tension discs — two metal plates the thread passes between. Lifting the foot opens the discs; lowering it closes them. Thread the machine with the foot down and the thread simply lies over the top of the closed discs instead of dropping between them, so it has no tension at all.
Do not just re-seat the thread. Pull it all the way out and start again: raise the presser foot, raise the needle to its highest point so the take-up lever is at the top, and follow the numbered path on your machine, making sure the thread goes down the right channel, around the bottom where the discs are, back up the left channel and through the eye of the take-up lever. Missing the take-up lever produces the same symptom. Full threading walkthrough.
2. Check the bobbin is loaded the right way round
On a drop-in bobbin, the thread must come off in the direction printed on the machine — usually counter-clockwise. That direction determines whether the thread runs through the bobbin tension slot or bypasses it. Every Brother machine we cover and the SINGER Heavy Duty 4423 use a drop-in bobbin.
On a front-loading bobbin — the SINGER M1500 uses one, taking a Class 15 transparent bobbin — the thread must pass through the slot in the case and under the flat tension spring. Test it: hold the case by its thread and it should hang without unwinding, and drop slowly with a small jerk of your hand.
3. Check the bobbin itself
Two things go wrong here. A loosely wound bobbin feeds unevenly, and that happens when the thread was not taken around the bobbin winder's separate tension disc while winding. Rewind it.
The wrong bobbin class. A bobbin that is fractionally too tall, too wide or made of the wrong material sits at an angle and feeds unevenly. Singer publishes Class 15 transparent for both machines we cover and specifically recommends SINGER or INSPIRA-branded bobbins for best results. Your manual states yours. A bag of assorted bobbins from a bargain accessory kit is a genuine, frequently-overlooked cause of this exact symptom.
4. Check for lint in the bobbin race
Remove the needle plate if your machine allows it, take the bobbin out and look. Sewing sheds lint — short fibers from the fabric and the thread — and a compacted pad of it under the bobbin changes how the whole assembly seats. Brush it out with the small brush your machine came with. Do not blow into it: your breath carries moisture, and pressurized air packs lint deeper into the mechanism.
The one thing not to do
Do not adjust the top tension dial to compensate. It is tempting: the top thread appears to be too loose, so tightening the dial seems logical. But the thread is not in the tension discs at all, so turning the dial changes nothing about this stitch — and it leaves your machine set wrong for the next project, at which point you have two problems and no memory of what the dial used to say.
If it is still wrong after all four
Change the needle. A bent needle sits fractionally off-center and the hook misses the loop, which can produce a similar mess. A needle that is not pushed fully up into the clamp does the same thing. Both are free to fix and both are step two of the general troubleshooting sequence.
If the machine sews a perfect stitch on scrap after rethreading and misbehaves only on your project fabric, the problem is the fabric-needle-thread combination rather than the machine: too heavy a needle for fine cloth, or too light a needle for thick layers. The needle guide covers matching.
Answers
Questions people ask
Why is my bobbin thread bunching up underneath?
Should I adjust the bobbin tension screw?
Why does thread bunch only sometimes?
Can the wrong bobbin cause thread bunching?
Do I need to clean my machine to fix this?
Where this came from
Sources
About the author
Scooter M.
An enthusiast who is genuinely into this. I read the manuals, compile the published specs, and do the arithmetic. No lab coat, no test bench, and no machines I am pretending to have used — which is exactly why every claim here carries a source.
Published by Slate Mill Group. Published September 1, 2026; last updated September 1, 2026. Editorial policy.
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