I had this same problem with our new rat. He couldn't get enough water from the bottle so my husband squeezed some out and then he lapped it up off the plastic ledge that led from the ladder, so we got a heavy crock and filled it with cold bottled water (put in an ice cube to cool it), and he drank like he hadn't seen water in forever.
If your rats are eating a diet with a lot of water content they might not drink as much, like if you feed them little bits of watermelon, or baby food (carrots, apples, plums and so on), they wouldn't need as much.
Still, rats can drink as much as two ounces a day, and ours drinks a lot, even when we give him his goodies in baby food form (it helps with the bonding--I feed him from a spoon, after calling his name so he'll come to the front of the cage); he knows his name already--just five days after getting him. I work from home and talk to him A LOT.
If you use tap water, let it run for a while because of lead, chlorine and other toxins, and see if making it extra cold helps.
Hope any of that helps.