Lint in the shuttle race, wrong thread tension, bad needle timing, a damaged needle or using the wrong type of needle can cause a sewing machine to skip stitches.
Use the right size and type of needle for the fabric and thread you're using so the needle can hook the bottom thread and doesn't skip stitches. Check the needle for wear and damage. Replace a dull or bent needle that won't pick up the bottom thread.
Check thread tension because the needle can skip stitches if upper or lower thread tension is too tight or too loose.
Check the shuttle race for lint buildup because the debris can prevent the needle from hooking the bottom thread. Clean the shuttle race if necessary.
The needle also will skip stitches if needle timing is off. Adjustment of the needle timing typically requires service by a technician, who uses special tools to synchronize the shuttle hook with the needle. If you eliminate all possible causes for skipped stitches except for bad needle timing, you'll likely need to have a service technician adjust the timing.