How old is your tank, what other corals are present and how many monti's have died? If the tank is older even running carbon the other corals may be inhibiting the monti with allelopathic chemicals.
What lighting were the monti's under before you got them - too much ligh or too little as a change?
I was thinking along these same lines as above comment - Montis are pretty forgiving for sps corals and can tolerate a range on conditions, but not allelopathy.
Acclimation can also play into this. How did you acclimate the montis (all of them)?
in general soft corals can make it pretty tough to keep hard corals without a lot of space and or carbon and/or water changes. What soft corals do you have, how big is the tank. when you acclimatize how long/what is the process you use?
I have had some that don't care where in the tank i put it and others that no where in the tank is good. water flow is another factor to keep in mind - was it under high/low/turbulent prior to you getting.
I have seen some tanks that are well established become immunized against certain genus/species but a lot of the above may be more important to get more info on.