9960 machine is not tightening the back side of the

What causes eyelashes when sewing?

The term eyelashes is used when there is an extreme case of looping of the threads on the back or top of a quilt. The typical response to eyelashes is to blame the thread for causing the problem.

https://www.superiorthreads.com/education/eyelashes-when-sewing

Check for rough patches and burrs everywhere the thread passes... use a fine emery board to polish

Tease out a QTip/Cotton bud etc to be a little fluffy, then trace all over the metal and plastic regions and the bobbin case… it will likely catch and show you where the burr is

Use quality thread... old thread can become weak and perished

This is generally applicable to the situation, loose under, yet if there is compacted lint/thread in the Tension Unit, it can have the effect of increasing tension

Lint in the tension of your bobbin case will cause it to lose tension and also needs care to clean periodically to avoid loose top thread

Looping ..??

It can cause an overload error too

Ensure the Thread is pulled firmly into the Top Tension Unit

ALWAYS raise Presser Foot when threading top... it relaxes the tension

If that is OK, this is likely to be lint in the Top Tension

It builds over time and keeps the plates apart

The discs are held open by thread or lint, they lose pressure, and the result is too much thread is delivered, and it bunches and loops like this, or, rarely, traps thread and becomes too tight

Easy fix tho' ..... simply 'Floss' between the top tension discs/plates, in the slot where the thread tension is adjusted, as you would your teeth

Use a strip of clean fabric (I use lycra for the stretch, but anything will do)

A small amount of alcohol on the cloth will aid the process immeasurably

It can take several applications to get it done, and in extreme cases, a small probe, like a tiny crochet hook, hat pin, mattress needle or similar, may be needed to clear the worst, then proceed as above

Same principle for Overlockers (Sergers) too

Once the Top Tension (and/or Bobbin Tension) is cleared of accumulated lint, the tension mechanism/s will revert to their standard range and the balance will return
It can take several attempts to clear it all

Wise to include the flossing as part of the general cleaning routine

The flossing method is a tried and trusted way to remove the lint accumulated in the Top Tension

This lint is the single most frustrating thing to address without this simple tip to alleviate the problem

In extreme cases, it may be necessary to dismantle the unit to clean it out

Best Wishes

You’re welcome