Frustrated w/training class--dog seems less obedient than before

[MomofSpitz]

Hello, all. First I'm new here so a bit of background. In Sept. we adopted a beautiful (albeit very skinny--not just our opinion but the vet's as well) 3 yr old German Shepherd. He already knew his primary commands of sit, stay, come, no, lay down, shake, heel, etc. The previous owner had gotten him from a breeder but was keeping him in an apartment crated much of the day and he just wasn't very okay with that. So that's how he ended up in the shelter and we have a large privacy fenced yard where he spends most of the day while we're at work.

Anyway, our daughter mostly is the one to take him for walks and as time wore on, he was less than cooperative about his "heel" command and in his over-exhuberance had a tendency to try and drag her a bit so we thought it might be good to get a refresher course for him and as much, for us. Granted we've only had one class but overall it was more of a frustration for me and by extension I can only imagine him as well. Before class he was very consistent with me on all commands except for the heel command. That was one we worked on in class as well and now not only is it difficult to get him to obey that one, he also seems to be paying less attention to the others that he was really good with.

So, I'm looking for advice because I'm sure that it's something I'm doing wrong. One thing I should mention in case it helps is that we had been giving him just voice commands most of the time and sometimes he got a treat if we were working with him specifically on training and running him through a series of commands but otherwise for just obeying he got a "Good boy!" "Good dog!" and petting if he was in reach as a reward. At the class, they are using treats and a closed fist that might or might not get a treat to ellicit the correct response (although it's an intermediate class so they're asking us to start only giving treats for a really quick response to the commands he already knows and consistent treats for new commands.) The thing is, we were already doing that and truthfully, after a short amount of time working with him, he gets pretty disinterested in the treats and it doesn't seem to motivate.

Anyway, I apologize for the long post and welcome any advice/feedback even if it's just to tell me that it's normal for the dog to backslide a bit with starting into new training and that it will get better. I was only concerned because it wasn't just in class (where he is definitely distracted by the other dogs and the shoppers outside of class --it takes place in a pet supply store) that he seemed less responsive but was also that way at home in the few days afterward.