Recall (come when called) can be hard to train, and unfortunately really easy to mess up. Some dogs have particularly fragile recalls, and with these dogs it's even more difficult to keep them reliable about coming when called.
Here are some suggestions for how to keep your dogs ability to come when called in tip-top shape!
1) Don't drill long distance recalls. As your dog gets better and better at coming when called, the temptation is great to set it up very far away and then call. Oh, the beauty of your dog racing at speed to return to you! So you do it again...and again...and again.
With each successive recall, the dogs stride gets a little shorter. That gallop turns to a trot, then to a walk. Your dog is getting tired! It's fine to practice multiple recalls in one training session, but keep the distances mostly short.
2) Be conscious of what you are asking your dog to recall away from. "Come" in your living room with nothing else going on is worlds different from "Come" in a park with a squirrel running by. Your praise and thanks to the dog should reflect how challenging it was.
3) Mind your reinforcers. There are some dogs, usually highly independent dogs of highly independent breeds, whose owners may choose to *always* reward coming when called, and never go through the process of weaning off treats. This is actually an acceptable decision, as the recall is SO important, and for some breeds it really is difficult to train to absolute 100% reliability. However, if this is your situation, then make sure to always have your dogs favorite reward handy!
4) Monitor tiny variations of the recall, and solve problems that creep in before they get too bad.
Maybe your dog was originally doing that awesome, turn on a dime and full-speed gallop to hand (the "official" K-9 Prodigy recall.) As the months go by, you notice that now he's not hitting his target quite so well, but you shrug it off, because at least he came back. A few more months go by and now he's sometimes missing the target altogether. You let that go too because he still looks better than all the other dogs in the dog park. Be really careful with this kind of thing! If you keep letting things go because the dog is "kind of" doing it, it's a slippery slope to slowly the whole thing falling apart, and you'll never even notice at what point it actually failed. The best thing to do is to monitor the quality of the recall (each recall) and do a quick review (quick reviews are usually 1 or 2 minutes, they really are quick) if needed.
5) This is common knowledge and should go without saying, but if you need to access your dog to do something unpleasant to it, like get a bath, just go get your dog. Don't make him wish he'd never obeyed you!