Lower Bobbin not threading

Knotting occurs when your top thread and bottom thread are not properly contacted or do not interact correctly. Whenever knotting happens, you will notice that thread will bunch underneath the fabric. You can try re-threading the upper part of your sewing machine, making sure the thread is passing through every single thread guide on its way to the needle. If you find yourself constantly adjusting the tension of your bobbin thread, check the bobbin itself. Plastic bobbins especially can wear, causing them to grow loose within the bobbin housing and making it difficult to maintain correct tension. Try switching to a new metal bobbin and see if that resolves your issue.

Knots? Bird nests under the fabric?

Remove the top thread. ALWAYS RAISE the presser foot. Rethread from the beginning. Verify the thread path is correct.

Birds nest under the fabric big loops of thread top side looks good...

Sewing Machine Thread Bunching Up Here Why

What causes Bird Nests Superior Threads

Please check your fabric results. Is it the bobbin thread that is accumulating under the fabric, or is it really the top thread that is globbing and knotting under the fabric. Usually, thread mishaps under the fabric are the result of an issue with the top thread.

ALWAYS RAISE the presser foot when threading the upper thread. This releases the tension disks so the thread will seat properly. Use a brand new needle. Start out with the upper thread tension set midway between high & low numbers, although this can be tweaked.

If it is in fact the upper thread, then here are some suggestions:

Sewing Machine Thread Bunching Up Here Why

Troubleshooting Thread Nesting Sew Toot

Usually, thread bunching under the fabric is an upper tension issue. Be sure to ALWAYS RAISE the presser foot when threading the upper thread.

With the bobbin case operating at a tilt, there is something mechanically wrong with the machine. It may be best to take it for service.

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You didn't explain the symptoms. Is the thread bunching, looping, knotting under the fabric? Is the bobbin thread rising to the top of the fabric?

If it is bunching under the fabric, it is probably what is called thread nesting or bird nesting. There are several fixes, sometimes more than one fix is required to resolve the issue.

Be sure to clean under the needle plate, around the bobbin, and the feed dogs. If the machine has instructions for oiling, follow the directions using a fresh, good quality sewing machine oil and apply only 1-2 drops each spot.

Install a brand new needle. Make sure you are using a needle compatible with the thread and fabric being used. All About Needles
Best practice is to use the same thread in both the upper and bobbin. Also, avoid old or bargain bin thread. Spend an extra couple of dollars to reduce frustration and increase success with your machine. If you wish, you may use two different colors of thread to test so you can easily determine what is happening with the tension.

Remove the thread from the machine. RAISE the presser foot and rethread from the beginning--the thread needs to be seated firmly in the tension disk. Verify that the correct thread path is followed (consult the owner's manual).

Set the upper tension to the midway point between high & low numbers (this is the factory standard and is a good place to start, but every machine is different and may need adjustment).

Confirm that the bobbin is installed in the bobbin case (turning the correct direction) and that the bobbin case is threaded properly.

Before beginning to stitch, hand rotate the handwheel one rotation and pull the bobbin thread to the top of the fabric. Then, gently hold the thread tails in the left hand and slowly begin to stitch.

Test.

If thread is bunching, looping under the fabric, the upper tension is too loose. Tighten it and retest. On most machines, the lower the tension number, the looser the tension.

If the bobbin thread is rising to the top of the fabric, the upper tension is too tight. Loosen the upper tension and retest.

Keep adjusting and testing until it is where you want it. Generally, tension is correct when both threads (upper & bobbin) meet in the middle of the fabric. FWIW, tension is static and should be tested and readjusted every time a new project is begun. The needle, thread, and fabric will affect the tension, so it is important to get comfortable adjusting the tension.

Also found the following reviews. Appears that some users have had tension issues with their CG550.

Amazon com Customer Reviews Singer CG 550 10 Stitch Commercial Grade...

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Sounds like bird nesting. The upper thread accumulates under the fabric creating a series of loops and knots.

First: Install a brand new needle and make sure it is installed facing the correct direction. (A new needle is the least expensive fix for most sewing machine issues.)

Second: Remove the thread. RAISE the presser foot and rethread from the beginning. Make sure it is threaded correctly (consult the owner's manual).

Third: Pull up the bobbin thread through the fabric and gently hold the thread tails in your left hand while slowly taking the first couple of stitches.

How and Why to Bring up the Bobbin Thread

Sewing Machine Thread Bunching Up Here Why

https://answers.yahoo.com/question/index?qid=20100303212248AAnRUQA

Why Do Get Bunched Up Thread Under My Fabric

WHY IS THREAD BUNCHING UNDERNEATH MY FABRIC

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The tension on the bobbin is too high, or the tension on the top is too low. One needs to set them to balance
Bruno

I have the same problem but i have not watched it. I does not seem that anyone wants to help us here, I am going to other sites until i find someone knowledgeable. apparently we are the smartest here at the moment ;)

try changing the needle --make sure the needle is the right one
for the fabric being sewed
try a different thread
make sure the bobbin is not over filled
make sure the bobbin is turned the right way in the bobbin case
make sure the thread has not come off the thread take up lever

bunching lower thread is actually the upper thread not having enough tension due to mis-threading or wrong tension setting