Different machine types, different hooks.

Sewing machines can either have a rotating or an oscillating hook. For both hook types observing the timing and resetting it, if necessary, follows the same principles.

Exactly what is timed!

A needle and hook are "timed" to pass by each other as a loop is forming. The clearance between the needle and hook as they pass is 0.4mm or less. The timing is set very accurately during manufacture. An incorrectly timed machine will not sew.

Exactly how to reset the timing!

Normally the timing either cannot be changed or if it is adjustable, it has been changed by misadventure! Understanding how to check if a machine's timing is correct is a first step. Adjusting it is a second step, but only if needs to be adjusted. Knowing that it is correct may be helpful in sorting out other much commoner stitch formation problems unrelated to timing.

Helpful timing tips:

Tip 1. If the lower thread "picks up" easily when the machine is being threaded for use, then the timing is likely to be just fine. Forget the whole timing thing and find the problem elsewhere.

Tip 2. Adjusting the timing when you have definately measured it as being incorrect, usually involves moving the position of the hook relative to the needle. This varies with machines and may require confidence, observation, perhaps a service manual or specialist knowledge for that machine.