I have 4 return jets in my built-in spa. When I set the return to spa-only mode, 3 of the jets shoot water really hard and aerate properly. The 4th jet just barely has a small trickle of water drip out of it. As far as I can tell, there is no noticeable water loss, so I doubt it's a leak. Perhaps a clog? What is the best way to go about troubleshooting this problem?