Hamsters drink very little, as they are native to the desert, their bodies don't need much moisture. My biggest hamster drinks less than a tablespoon of water per day. My smallest drinks so little it takes several days for the water level to drop any noticeable amount. Even if you monitor the bottle carefully, it can be difficult to notice such a small amount.
Also, I didn't see either of my hamsters drink anything until about 3 days after I had brought them home. Probably because of stress from being moved. It's possible they did drink during this time but I didn't see it.
You can do a quick check for dehydration by gently pulling back the skin on the scruff of their neck (you can search the forum for threads about scruffing a hamster to see what I mean) and then letting go. On a well-hydrated hamster, the skin should spring back into place immediately. If it doesn't, there's some dehydration occurring.
hope this helps.