I've had my Fender Blues Jr. for more than 6 months now, and I guess around the 2 or 3 month mark, the spring reverb unit stopped working.

I didn't really question it because I had a marshall reverb pedal that could pretty much fill in, but now that I remember how the subtle little "splashes" of the spring reverb sounded when I played added to my tone, I want it back.

Does anybody know what the problem could be? I've checked the springs and they seem to be fine, the tubes are all in working order. Could this be a wire issue?
The spring reverb went on my FBJ within a week of buying it, it's a common fault. Beyond checking the RCA jacks and valves I never bothered to investigate - if it's still under warranty just take it back. Don't tell them how long ago you broke it though

If for some reason return is not an option, try and localise the fault. Swap the reverb driver tube for a known good one ( can't remeber which one in the Junior, V2 I'd assume). Give the tank itself a sharp tap while the amp's running - if you get that horrid metallic crashing sound the tank itself should be fine, so the signal routing is the likely culprit. Also remove the RCA jacks at each connection point and reattach them - sometimes they can oxidize and prevent proper conductivity. If no noise comes from tapping the tank but you get a buzzy grounding noise or hum when touching the tip of the reverb return lead, the tank itself is at fault. Try borrowing the tank from a mate's amp (any accutronics type will do).

I sold my blues junior months ago so i'm working from memory, but hopefully the steps above will help you determine where the fault lies.