Incorrect upper threading is the most common cause of bobbin thread showing on top. Even if you've already done it, go through the full process again slowly, referring to your manual rather than going from memory.

• Raise the presser foot before re-threading so the tension discs open properly
• Follow your machine's threading path carefully, making sure the thread passes through every guide and tension disc in the correct order

Upper tension that's too loose means the upper thread isn't pulling taut enough to lock the stitch in the middle of the fabric, so the bobbin thread gets pulled up to the surface.

• Increase the upper tension gradually and test on scrap fabric after each adjustment
• If adjusting the tension makes no difference at all, there may be lint or a thread fragment stuck between the tension discs, preventing them from closing properly and leaving you with effectively no tension
• To clear this, try flossing a piece of thread or a thin strip of fabric through the tension discs and pulling it through - this can dislodge whatever is caught in there. Lower and raise the presser foot a couple of times as you do it to open and close the discs

While the upper thread is usually responsible, a poorly set up bobbin can also contribute to tension imbalance.

• Remove and re-insert the bobbin, making sure it's going in the correct direction for your machine
• Make sure the thread is pulled through the tension slot properly
• Check the bobbin is wound evenly - an unevenly wound bobbin can cause inconsistent tension
• Make sure you're using the correct bobbin type for your machine

If adjusting the upper tension hasn't resolved the issue, the bobbin tension may be too loose. This should always be treated as a last resort.

• Refer to your machine's manual before making any adjustments to bobbin tension
• Make very small adjustments and test on scrap fabric after each change

A damaged or incorrect needle can affect how the stitch forms and cause the bobbin thread to pull up to the surface.

• Replace with a fresh embroidery needle
• Make sure it's the correct type for your fabric and fully inserted with the flat side to the back

A significant difference in weight between the upper thread and the bobbin thread can cause tension imbalance that's hard to dial out with tension adjustments alone.

• Check that the thread weights are compatible for the type of embroidery you're doing
• Most machine embroidery uses a lightweight bobbin thread paired with standard 40wt embroidery thread on top - mixing weights outside of this can cause issues

Before returning to your project, run a test on scrap fabric of the same type. Check the top and underside - the bobbin thread should not be visible on the surface and the stitches should look clean and even on both sides.