Hamsters don't drink an awful lot in one go, so when looking at a big water bottle it can often look like it hasn't gone down despite the fact your ham will almost definitely be using it. They will probably be drinking from it mostly during the night when you are asleep too so you might think you never see them drinking from it, when they probably are.
To put your mind at rest about the amount of water your ham is taking in, put some cucumber in their cage as this has lots of water in it which will also help rehydrate your ham should they need it.
Finally, just to be sure, check your bottle is definitely working and water flows freely through it when you put your finger on the end. You could also try rubbing some cucumber over the spout of the bottle to tempt your ham over to it. But in all honesty, if your bottle is working then I'm sure it's being used