First, Never trust any information on fishkeeping from a petstore. This is unfortunately a sad truth where I'm from (canada) and as I've heard also in the US.
So your betta is "hiding" and not eating. Beside that, no health issues visible ?
Not eating is a symptom of a serious problem with the fish. Either physical or emotional.
What food do you feed and how much? Was it eating before? Are the other fishes taking all the food before it ?
Bettas are calm fishes. They will not race all the time, they like to spend at least some time completely immobile, hovering there. (not floating nor sinking). If there is too much water current in your tank, you betta might find a more quiet spot and stay only there. That could be a reason.
You have way too many fishes in there in my opinion. The betta could be alone in there and fine. The cories.. ok.. I guess... But then you add a lot of loaches, starting to be too many fishes in there. And the guppies are not a good idea. It is well known that a betta will often be aggressive with guppies. Overall I think there are way too many fishes in that 10 gallons. If your betta feels he doesn't have a territory of his own, yes he will hide in a small corner and become lethargic. So that might be the source of your problem too.
Please update us with your readings on pH, ammonia, nitrites, nitrates, as advised by comments. Tell us what you feed and how much. And tell us how often you change water please.
Good luck !