Incorrect upper threading is one of the most common causes of bobbin case lifting. Even if the machine looks threaded correctly, a full re-thread is always worth doing first.

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

The take-up lever is the moving arm that pulls the thread up with each stitch. If the thread isn't seated in it correctly, tension suffers and the bobbin case can lift as a result.

• Confirm the thread passes through the take-up lever, not around it
• This is easy to miss - if you're not certain, re-thread completely rather than guessing

Just above the needle there's a small thread guide that's easy to overlook. Skipping it - or having the thread slip out of it - can cause uneven tension and contribute to the bobbin case lifting.

• After the thread exits the take-up lever, make sure it passes through this last guide before going through the needle
• Check it's sitting snugly in place and hasn't slipped out

A bobbin that isn't sitting properly in the case can cause the whole assembly to move around during sewing.

• Remove the bobbin and re-insert it, making sure it turns in the correct direction for your machine - check your manual if unsure
• Pull the thread through the tension slot on the bobbin case
• The bobbin should feel firm in the case with no wobble

The bobbin case itself needs to sit securely in the machine. If it isn't locked in place properly, it will shift during sewing.

• Remove and re-seat the bobbin case, making sure it clicks or drops firmly into position
• If your machine has a latch, locking tab or stopper, confirm it's fully engaged
• The case should feel solid with no movement when nudged gently

A cracked, dented or warped bobbin case won't sit flat and will rock or lift as the machine runs.

• Look closely for any cracks, chips or visible warping
• If you spot any damage, replace the case - they're relatively inexpensive and straightforward to swap out

The needle plate sits directly above the bobbin case. If it's loose or slightly misaligned it can allow the bobbin case to shift during sewing.

• Use a small screwdriver to check and tighten the needle plate screws
• Even a small amount of movement here can cause problems

Lint and thread fragments can build up in the bobbin compartment and stop the case from sitting level.

• Use a small brush or vacuum attachment to clear out any lint, thread bits or fabric debris
• Pay attention to the area around the bobbin case and feed dogs

Once you've worked through the steps above, re-thread the machine, load the bobbin and sew a few lines on scrap fabric. Watch to see if the bobbin case stays in place and check that stitches look clean on both sides.