Just bought s Singer 1725. Tried a straight stitch. The thread keeps bunching up under the fabric and around the bobbin shuttle. How can this be corrected?

Likely the bobbin thread tension is too low. You can verify it is bobbin thread by test sewing with a contrasting top thread color. If it is bobbin thread only that is bunching on the bottom side, increase bobbin tension by tightening the bobbin case tension spring. See instructions in operators manual.

Check your stitch sample. Dollars to donuts it's the top thread bird nesting (also called thread nesting) under the fabric. Issues appearing under the fabric is usually top thread related. Problems appearing on the top of the fabric is usually bobbin thread related.

Remove the top thread from your machine.
Be sure to start with a brand new needle.
ALWAYS RAISE the presser foot and rethread the top.

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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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Most likely, it's because the top thread is not seated fully in the tension disk.

Install a brand new needle.
Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning. Verify the thread path is correct.

FWIW, problems appearing under the fabric is usually a top thread issue. Problems appearing on top of the fabric is usually a bobbin thread issue.

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try changing the needle--use a needle according to the fabric
being sewed
if thin fabric is being sewed try using a stablizer
rethread the machine top and bobbin--use every thread guide
make sure the bobbin is inserted correctly and turned correctly
in the bobbin case
also try adjusting the top thread tension just a little

Change needle-flat side to the back of the machine
Clean under needle plate where feed dogs are for lint
Clean bobbin area for lint or loose threads
Rethread machine top thread make sure to get every thread guide
& make sure bobbin is wound correctly & that it is inserted correctly in the bobbin case or shuttle
Check tension 3 or 4 is usually good

with some machines the thread comes off the thread take up lever

bobbins in backwards
flip it over

Find your zig zag width control and set it at 0 You will have a straight stitch.

sewman7

Check the position of the bottom piece that holds the bobbin carrier to the left so it rides smoothly on the rotating hook that carries the upper thread around the bobbin. I found my wife"s machine loose so the bobbin carrier was out of position and not riding on the ledge of the rotating part. loosen two screws and push it to the left and tighten.
If upper thread bunches below the material it needs more tension so it pulls back up through the material.
If the old singers were fords this is a yugo
(been fixing and adjusting her machines for years. ))

I have a Singer 2950. The thread keeps getting bunched up on the reserve side of the fabric, and does not stitch. It is driving me crazy. I think I have the bobbin threaded correctly, as well as the machine. I have tried everything! Please help!