Sewing machine skipping stitches? It is almost always one of these five things

1. Needle. Bent, dull, or wrong type. Change it. Needles are consumables, swap every 8 hours of sewing.
2. Needle inserted backwards. Flat side goes to the back on most home machines.
3. Threading. Rethread top and bobbin with the presser foot up so the thread seats in the tension discs.
4. Lint in the bobbin case. Pull the needle plate, brush it out, one drop of oil where the manual says.
5. Timing. If 1 to 4 did not fix it and the hook is not catching the thread loop, the timing is off. That is a repair you can do with a YouTube video and patience, but it is the only one on this list that is.

Old all-metal machines from the 60s and 70s are worth fixing every time. New plastic ones under $150, often not.