Alot of Rugosa's are semi double.
SK, I'm surprised you thought they were all single petaled.
PP, I don't prune them much.
Just prune dead wood in the spring and
every few years I cut out some of the old wood.
I'm having problems with some of my Alba's doing as you described.
I would try cutting them way back.
Here's an Austin Rugosa 'Wild Edric'
