Not all pharmacies can bill Medicare Part B. CVS and Walgreens can, others like Rite-Aid and grocery pharmacies subcontract the work out to Advanced Diabetes Supply. While the companies can bill Part B, finding a pharmacy staffed with people who know how to do it, as you found, can be tough. You might need to visit multiple pharmacies around you to find one with good staff.
Last time this came up CVS inside Target was recommended because they are less busy. A regular CVS should work. Walgreens is a pharmacy of last resort because they are also going out of business. If you are using Humalog or Lispro there are some DME mail order companies that can also supply your insulin. Here’s a topic from last year the covers everything about Part B paying for insulin