Couple things to check, to confirm you have personal trained to ARFF equipment?
The call you are on now is within proximity of the ARFF equipment?
You have the ARFF expansion turned on?
you dont have more ARFF in your AAR then you actually have. (this one I have done before. only have 4 ARFF, typed in 5 rigs)
those are just a couple of things you could try, not trying to insult your intelligence or anything just a few things I could think of.
I checked. I have the training. Ran missions with before. I made units unavailable then available again and it seemed to fix it.
so again it is acting up. No ARFF units available. Only allows me to dispatch 1 MCI unit.