Why your sewing machine is skipping stitches

A stitch forms when the needle carries the top thread below the fabric, the thread bulges into a small loop, and a rotating hook passes through that loop to wrap it around the bobbin thread.

A skipped stitch means the hook arrived and the loop was not where it should have been. Understanding that makes the causes obvious.

The needle, first and almost always

Blunt. A blunt needle pushes fabric down instead of piercing it, which changes where the loop forms. Needles go blunt long before they look blunt. Change it as the first action, every time.

Bent. Even slightly. A bent needle passes the hook at the wrong distance. Any needle that has hit a pin is suspect.

Wrong type. This is the big one on specific fabrics. A universal needle deflects in heavy denim and cuts the loops in knit fabric; both cause skipping. Use a denim needle for denim and a ballpoint or jersey needle for knits - the needle guide has the whole chart.

Wrong size. Too fine for the fabric and the needle flexes under load.

Not pushed up far enough. A needle not seated to the top of its holder sits low, and the hook passes above the loop. Loosen the screw, push the needle up as far as it goes, retighten.

Fitted backwards. On almost every domestic machine the flat side of the shank faces the back.

The other causes, in order

Skipping on specific fabrics

Denim. A 100/16 denim needle, a slower speed, and the hand wheel over the crossing seams. See machines for denim and heavy fabric if it is a constant problem - a light plastic-bodied machine has limits no needle fixes.

Knits and jersey. A ballpoint or stretch needle. A universal needle pierces the knitted loops rather than sliding between them.

Very fine fabric. A finer needle, a shorter stitch, and a piece of tissue paper underneath to give the feed dogs something to grip.