What model/injection system are they?
I would disagree that cummins are hard to start in the cold. I have a few 6B's that I routinely start without their grid heaters at below freezing with almost no cranking time.
These engines do not have glow plugs, they occasionally have grid heaters but that should not be the problem this time of year.
Is the engine cranking over quickly? Do you still have full power? Can you notice any difference in how it runs?
My first suspicion would be a small air leak in the fuel system as suggested above. To test this, manually turn on/pump the lift
pump for a while before trying to start the engine. If the engine starts right away, there is most likely an air leak. There are other possibilities as to what is wrong but this is likely the culprit and it is easy to test. Finding the leak is a bit harder though.