zero key on num pad won’t work in dos

when using a dos program, the zero key does not work on the num pad. It works in any other program in windows. I have to use the zero key above the alpha keys. I have changed keyboards to test but with same results. I have noticed that occasionallythe zero key may work just once in a session on the num pad but is a rare occasion. I am no expert, but no how to prod around with properties & settings but can’t find any resolution. Can anyone give a suggestion ?