My bobbin thread is getting tangled underneath when I sew.

Philippa's Check-list:

Re-thread top and bottom. Hold on to top thread and give a tug before threading through the needle to ensure the thread is properly between the tension discs.

Make sure bobbin is wound cleanly - even, no crossing of thread, no loops or tails visible. It will not feed smoothly if it is not wound smoothly and don't over fill it as it will stick in the bobbin case.

Make sure you are using bobbins specifically designed for your make and model of machine. A cheap packet of extra bobbins will probably not fit!

Change the needle, it might be blunt or damaged.

Try a bigger needle size.

Make sure the top thread is feeding off evenly and smoothly - not catching anywhere or sticking.

If not chaining off or using a leader strip at the start and end of each seam have long thread tails to hold on to to stop them snarling.

Try 100 weight Superior Threads Kimono silk in the top and Bottomline in the bobbin. Usually sews like a dream.

Sew at a consistent speed with no sudden surges of speed that result in a demand for thread at an uneven rate.