This is why I visually verify that each chicken is drinking before I permanently pull alternate water sources - I've also had a bird that struggled to use the horizontal nipples, and it took maybe 2 weeks for her to really get the hang of it (she knew to go to the nipples, but would wait for someone else to drink and then steal any dripping water).Last year, I took out the regular open waterer when I saw that my 3 pullets had figured out the horizontal nipple. My cockerel didn't figure it out. I saw him trying to bite the whole red plastic nipple, and figured he was on his way to getting it right.
Later the second day, he seemed lethargic and I though he looked skinnier...?!? It dawned on me he was dehydrated!
I put out the other waterer, and he went right to it and tanked up.
If I ever try to teach them him HN again, I will put the open waterer out for an hour before dusk to make sure everyone gets water before roosting.