JRTs are notorious for acting like that. They're very tightly wound as a breed. With that said you can change any dog to conform to any behavior you desire. Thundershirts are more of a band-aid being put on a bullet wound at this point. If you are constantly putting the Thundershirt on everytime your parents leave you're not addressing the root of the issue. Thundershirts were invented for, surprise surprise, to alleviate dog stress during thunderstorms. It is very difficult to counter-condition a dog for a thunderstorm. You basically need to have your puppy and get 3-4 violent storms while they're still young to really make them not care about thunder and lightning. You can still do the same with older dogs but they'll probably still be nervous during them. But now people are applying the Thundershirt for all forms of dog stress. Does it get short term, immediate, results? Yes. Is it fixing the problem? No. No dog is ever "too far gone." Sure old dogs can be stuck in their ways and it takes longer but you can eventually shape their behavior. Like Arkk said my guess is your parents always made a big deal about leaving and a big deal about arriving home. Ideally you should just say "I'm leaving" to the dog and walk out the door. No petting, no kissy faces, no acting like it is the end of the world. When you arrive home you walk in and ignore the dog until it calms down. Reggie gets the silent treatment until he puts his ears up and doesn't wave his tail like a maniac everytime I make eye contact with him. He now will go into that mode in less than a minute now after working on it for about a month now.
Now to come full circle with all that said. Thundershirts do work *for most dogs.* Some dogs don't care for them and some dogs think it is the most comforting thing in the world. There is no guarantee it will work.
