While there are a handful of different timings in a sewing machine, the hook timing is the most common to cause a sewing machine to not function correctly. Hook timing is the relationship of the hook tip and the needle eye. If you think your sewing machine may have a timing problem, a careful examination of the hook and needle can confirm that condition.
When the needle is moving upwards, it creates a loop behind the needle, between the needle eye and the fabric. The hook tip must pass through that loop in order to create a stitch. There is a fine tuning of the hook timing, depending on the brand and model of the sewing machine, but nearly all sewing machines adhere to this basic principle.