Late to the party on this one as well, but if the original contactor had a NC auxiliary contact and the circuit is single phase, then one of the main contacts has been used as the aux to hold the contactor on, and as already said, the fault will probably be with a safety circuit external to the contactor.
I'd be getting a pen and some paper and tracing out the circuit of a) what I have with the faulty machine, and b) what it should be, from the adjacent, working, and hopefully, unmolested machine, if it is identical.