2 things to check right off:
Cables to and from spring tank. Try re-seating them to ensure they have a good connection. Cables can and do go bad; try a spare(s) if you have one.
Try a different tube in reverb driver position (refer to manual as to which one this is).
These are quick and easy checks for the casual user. Other things can cause the issue, but usually require a technician to troubleshoot.
Also the spring tanks can fail as well (not too common). Do you hear the springs when you tap on the tank?
Be aware that this alone does not absolutely mean the tank is bad, but if you hear the spring noise, at least the tank's output is getting through.
In any case, good luck to you in getting it resolved.