Check your glow plug timer relay. Neileg is right in that if the timer keeps the plusg on too long they will burn out for sure. I have 2 Chevy Diesel trucks that eat their share of glow plugs. Some times they just burn out at odd intervals and sometimes the timer is at fault (not often though).

You could test each plug circuit with an amp guage (I think they draw about 5 amps each), let the system cycle a couple of time and see what each plug draws. If the circuits are not the same you have a problem in the wiring between the timer and the plug I would think.