You need to give them a week or so atleast, could just be an oversight and they could be busy.
I can go days without picking up messages if I'm up against it.
I would give them time to respond but ultimately even if they don't get back to you it's likely that the manufacturer will register the warranty if you can prove it was installed professionally.
Don't try registering yourself tho untill you've given installer atleast a couple of weeks to do it for you.
I've got the same issue at the moment where I haven't got round to registering due to an oversight and being too busy, customer called but because I didn't answer they registered it themselves, now because I'm a registered installer it means they won't benefit from the priority callouts for a warranty issue that they would if they'd had more patience and I've lost some leverage with manufacturer for having less registered installs.
If it helps think of it like this, if you have a fault and no heating or a leak would you want your engineer to get back to you ASAP and rush to your aid or after they've spent an hour or so on an 18month old registration for a boiler with no faults?