They don't drink often and mine seem to mostly have a drink at night when they're most active - I occasionally catch them just as I'm going to bed. My guess is that you're just not around to see it.
I feed outside the viv and sometimes I quickly dunk the end of their snouts into the water bowl as I'm putting them back. This triggers drinking in some, so maybe you could give that a try?
If you continue to be worried, a sign of dehydration is when their skin starts to look wrinkly or saggy and they look a little shrunken in. This very rarely happens unless a Corn is sick to start with.