Did you pay them? How much?
What do you mean "when I discovered it"? What have you discovered since the end of last summer when they were there?
Because I don't know the details, I'm not sure what you mean by saying you didn't call them on it.
But are you sure it's too late? Did you pay them for an inspection and that payment was supposed to be applied to the cost of repairing it? Maybe you can still do that, but I think you actually have to find the problem. (Are there more than one?)
Do you just want a refund because they didnt' find the problem? You haven't found it either, so we don't have evidence there is a findable problem. I'm guessing you won't get a refund at all, and certainly not this much later, except maybe by the most dutiful company, and if they are that good, I'd let them do the repair.