Hot tub jets receive water outflow from the unit's pump, typically the hardest working piece of equipment in the hot tub. When jets begin to fail inside the tub, you most likely need to diagnose problems relating to the pump, not the jets.
Check on the Priming
You will need to make sure that your pump's priming is working properly. To do this, open the access panel on the hot tub and open the airlock valve near the pump. This will bleed air from the line and purge the system, allowing more water to flow through the jets.
Clean the Tub's Parts
Clogged water intakes, dirty or clogged filters, and clogged jets can also cause jet problems. Clean each of these items to improve water flow through the hot tub. You may need to replace the filter if you cannot clean it properly.
Reset the Tub
Turning power off to the spa and then turning it back on after a few seconds can reset the unit and correct pump problems. However, extremely noisy pumps or severe vibration can result in the need to overhaul or replace the pump.