Sometimes no glow light; new plugs already

Hey, I used the search and could not find the answer, but I am sure it's there. Every now and then the glow plug light will not come on. The plugs are all brand new, and the car will not start (indicating that one is not faulty). The problem first started in cold weather, but the engine was still warm (stopped for dinner). I have gotten the light to come on via repeated reset using the jey (off then on), but am worried that the problem might worsen and get me in the wrong place at the wrong time (e.g. during bank robbery). Any suggestions or references to another thread? Thanks.

RB
85 300Dt Sedan