Last month, I had same prob at ASDA! Went to Boots - no problem! This month ASDA had plenty so looks like a temporary prob. I now check with my pharm before ordering and if there is a prob, I source elsewhere.
There's no such thing as duty bound, I'm afraid, it's simple supply and demand. If the wholesaler has them, then they will order them. If there is no stock available they won't be able to. However other pharmacists may well have better stocks, particularly if you can get to a bigger one in a town.