Thanks tai! We found it: Trainer Frod's quest The Empty Pens sent you up the Pet-Walking Road without Bartos's letter, so at the top there was nothing to hand over and nothing came back. Starting the quest now hands you Bartos's letter, and its light bulb lights up after 5 minutes so you can hand it in. We also walked through the pet course talks with Bartos and Frod in every order. To check: start the quest, take your pet up the road with the letter in your Etc tab and talk to Frod at the top. If he still gives you nothing, tell us which step you were on.