Originally Posted by dunster

On a machine with a stitch regulator, the encoders determine the speed at which the machine is moving, and therefore tell the needle when to go up and down. If you have long stitches where the needle is not going down, then check the encoders.

However that is not your issue. Your needle is going down and not always forming a stitch. That can be several things, including a bobbin that is not wound tightly, needle bar height needing adjustment, timing slightly off, needle not positioned correctly or slightly bent, and checkspring needing replacement. I agree with others that you probably do not need a new hook assembly.