Don't know if my info will help since I'm in FL zone 9 but my A. currasavica ripened this past spring & summer. They grow year round here but I think the key may be that they didn't get a lot of water where they were planted so that may have triggered the maturity of seedpod.
The ones I have growing now get more water and haven't ripened.
Denise in Naples FL

I'm in zone 4 and my Asclepias Curassavica seeds are still ripening for me and we have had many a night of below freezing (many in the 20's). I check each day and pick a few pods. Even while I was out of town, my daughter picked a few pods a day (she watched for the splitting).

A lot of my perennial plants look bad after the freezes but they are hanging in there and ripening.

Don't give up.