I think that when young, plecos tend to be very fragile. I finally have three stripped bulldog (rubber-lip) plecos that are healthy and happy. This is in my 55 gallon tank. I ended up going to PetSmart a couple more times, with ultimately four plecos dying. I took a risk with my last two, I brought them home and just netted them from the bag to my tank; no floating, no mixing water. I figured the less time in the bag would be less stress. They were in the bag for 10 minutes only, and then were hiding under logs in my tank. Since my lfs is on the same city water as I am, the only diference between their water and mine is the temp and the bit of salt they use.
I think a large part of the reason is the stress of shipping. Most of the plecos I have to choose from have concave bellies, and I don't think they get feed much in the store tank. For the first time ever I saw a zucchini in the tank last friday. I think prior to that they were just sprinkling in flake food, and that was likely getting eaten by the angels they keep in the same tank.
My plecos love the zucchini I drop in the tank every two days. I occasionally see them chewing on the bit of algae on my bogwood and the sides of the tank. I've never seen them eat the algae tabs I use, but at night they are more active and I may miss it.
Don't give up on raising a pleco, they are great fish.