I had this exact same situation 2 weeks ago. I am myself a first time chicken owner and my 4 girls are about 10 weeks old. I did a little digging and found lots of people saying that chickens will take the water that is easiest to get, and so if their chick bowl is available, they will choose that over the nipple waterer every time. So one weekend day when I could keep a closer eye on them to make sure they didn’t get dehydrated I took away their bowl waterer, leaving them with only the nipple waterer. Sure enough, within an hour of me taking it away, they had figured out the nipple waterer and were using it regularly. It was a little nerve-wracking taking away a water source, but it worked like a charm. Two weeks later and they are using the nipple waterer like old pros.