I assumed that the premium stores have suppliers that are vetted by alidropship team.
I doubt it. How would they check if Aliexpress sellers are good message responders? They only check the order fulfillment, sales and feedback scores etc.
No one here or the Alidropship support can tell you why he won't respond.
1. Report him to Aliexpress support and hope that forces the seller to reply. (Don't hold your breath)
2. Change the supplier to another one with the same product if the idiot refuses to reply.
3. Do nothing.
I'd choose #2