My hermit crab really needs to change shells but refuses too. Can someone help me?? :S?

I got my hermit crab a year and a half ago at a local pet store and continue to get them from there because they seem to be in good health. When i got my hermit crab (Howie<3) , the petstore employees said that he would/should change shells pretty soon. That was a year and a half ago and he never changed. I'm beginning to get worried. He has been very lethargic for the last couple months and really needs to molt too. I have placed him in the perfect conditions to molt and he still wont dig down to do it. But my main focus is the shell changes. Two of my newest hermit crabs (Eve and Valentino) have been changing shells alot (everytime i try to bring a new shell home for Howie). I have 7 shells in the tank he can choose from ranging in all different sizes. Are there any hermit crab experts out there that can help me? Im thinking that Howie will be happier and less lethargic if he got/had a better fitting shell. All help is greatly appreciatied. Im striking out figuring out something else i can try. Pleasseeee HEEELP!!