What kind of "doesn't work" are they experiencing?
(not trying to be a smart-ass...there are variations on Not Working with fog machines)
Do they make any noise, but no smoke?
If so, what kind of noise? (hissing...loud, pulsating noise...humming...etc)
Are they getting warm, but no smoke? (at least that means the heaters are working)
Do they have a remote? If so, are there any lights showing on the remotes?
Without knowing any of the problems, I'm guessing they most likely have some dried fluid clogging the line (seems to be the most common problem).
If you don't have any fog machine cleaning fluid, try emptying the fluid vessel, and putting in a couple cups of a vinegar/distilled water mix (about 50/50)...try running it then (do so outside, however).
If that doesn't help, you may need to open up the case, and pull some of the innards apart to clean out the piping...there's a thread somewhere on this forum that's a great how-to on working on the inside of a fog machine...I'll see if I can find the link.
ok...i took the cover off two.....the pump on one seemd to be making noise....the other machine pump has a very very low hum....i took off both tubes to the fluid and made sure that they were clear....the machine pump that is making noise seems to not be pulling any fluid