1) What condition are the boots in? If they have lots of leaks, they may quit working
2) Wiring from the relay. I had this issue occur – the output wire broke off the relay. New one is $3,000. I replaced the broken part for $2.97.
3) Solenoid not working
The blue light will not illuminate unless you have attained a minimum pressure to the boots (at least that is how my plane is wired). This means that leaking cannot be the issue. Perhaps a blockage? I had leaking boots on the tail, although they would inflate (and the blue light illuminate) moist air was constantly pulled into the system. Once above the freezing level for a period of time the boots would stop working due to ice in the lines. Just a thought….