“If you don’t have anything nice to say, don’t say anything at all.”
It drives (some) Sellers nuts, but the cold reality is that not all Buyer’s agents respond to requests for showing feedback.
My standard practice as a listing agent (representing a Seller) is to make 2-3 attempts to find out how a showing went, especially the first 7-10 days a home is on the market.
However, there’s really no point in persisting beyond that.
Realtors rank pretty high on the 24/7 “accessibility scale” (see, “Getting a Realtor’s Attention“); if they have a Buyer with continuing interest . . . you’ll know about it.