I am not sure but will try to help.
My two year old wont go out in the rain to pee. but is pee pad trained too. but for a while he would not pee on the pad . I ended up putting the pee pad in a dog pen I sized just big enough for him to go in. We do this a lot when camping.Even if you can just put the pee pad in an area and block him in somehow he might go on it.
I told him to go and left him alone. He went.I am not sure if that would work for you or not. It is best to do it first thing in the morning when they have to really go and maybe give a treat after for going.You will get probably better advice on here from others as well
An exercise dog pen is so great to have if you can afford one.You can size them to any length or square big or small.
Good luck