Chickens are creatures of habit and don’t like change. Sometimes it takes them a while to get used to a change. Patience is often your friend.
Chickens tend to roost on the highest thing available. How high is the new roost compared to the old?
How old are they? Were they roosting before? My brooder-raised chicks normally start roosting around 10 to 12 weeks. I’ve had some start at 5 weeks, I’ve had some go much longer than 12 weeks, but 10 to 12 is a good average.
30 mm is about an inch and a quarter. That’s a little small but chickens have been known to roost on ¾” (19 mm) plywood. As well as how old they are, what breeds do you have? That will give a clue as to how big they are.
A few years back I played around with round tree branches, a 2x4 flat, and a 2x4 on edge. My conclusion is that people care about the shape of the roosts a lot more than chickens do.
Can you post a photo of your set-up and give us a bit more information. There’s not a lot to go on in your post.