I'm having the same issues (slow opening, partially opening, not cupping) this year. I'm glad to hear it IS just the cool weather this year and not something else going on in the gardens. I don't even dead head until mid-day or late in the day because it's hard to tell yesterdays bloom (because it is lasting longer). If I want anyone to go for a garden walk I try to make it in the afternoon as morning blooms are not that great looking.
I also have a lot of plants that are a LOT shorter than they should be. I have a suspicion it is from the "spring sickness". I seemed to have an abundance of that this year.
Sue