IME the Horizontal Nipples can take some time for them to 'get it',
especially with young chicks many just aren't strong enough to trigger them.
Vertical Nipples are much easier for them to use.
Here's my thoughts on 'nipple training'.
-First, it's good to know how much water your flock consumes 'normally', I top off water every morning and have marks on the waterers so I know about how much they drink.
Some feel nipples will not ever provide enough water and the birds will suffer because of it. By monitoring quantities consumed both before and after, I found they drank just as much via the nipples as an open waterer.
-Show them how with your finger(tho that might just train them to wait for your finger), and/or manually grab them and push their head/beak onto the trigger(easier with chicks than adults).
-No other water source, best to 'train' during mild weather when dehydration is less of an immediate health risk. I do provide an open waterer late in day to make sure they don't go to roost dehydrated, especially young chicks.
It can take days or weeks to get them fully switched over, just takes observation, consistency, and patience.