Check for any obstructions surrounding the jets. Small pieces of debris that you bring into the spa from outside can lodge inside the jet joints and prevent the jet from moving properly. Stuck debris will have to be removed with a brush or any other item that can reach the obstruction.
Check the water pressure. Some jets rotate via the force of the water flowing through them. Low water pressure will prevent these jets from rotating properly. Common issues that affect water pressure are a blocked filter and insufficient water in the spa.
Open the pressure valve, if your jets stopped working after you've just refilled the spa. Sometimes a refill can cause air bubbles to get stuck inside the plumbing of the spa. This can cause all sorts of problems, including jets that won't rotate. Opening the pressure valve will displace these air bubbles. This is clearly the problem, or at least one of them, if you open the valve and hear a hissing sound.
Adjust the diverter valve, located near the edge of the spa. This valve controls the operation of the jets and sometimes it inadvertently gets turned. This will affect the performance of the jets. Adjust the valve until all the jets are rotating and flowing water properly.
Look at the seals surrounding the jets. A broken jet seal will prevent the jet from rotating properly, and prevent the flow of water. The seal must be replaced if it is broken.
Call a repairman or your spa company's customer service if your spa continues to have problems. The problem may be complicated and require the assistance of a professional.