Working on the side as a mechanic with a buddy who has a shop, I do see both sides of this and understand.
Likely they are not driivng it enough to have the issue happen. In their time driving it..if the problem doesn't occur then it's tough for them to fix it. You meanwhile drive in many more conditions etc and notice the issue.
Take a look at what happened with my car. I noticed it jumped about 1,000 rpm and slapped back into gear. Was positive at 173,000 miles that either seemed like a slipping trans or maybe a bad 4th clutch hub. Ran the car for a year (13,000 miles) that way and it didn't seem worse and with no consistency. Swapped the trans out for a fully rebuilt unit with upgrades, samw issues only more often. This problem turned out to be a bad coil pack. Never showed a misfire, nothing else. And since I was the only one working on the car, building the trans and diagnosing, I know that no one gave me the run around.
Simply put.. be reasonable and they should return that favor. Too many people are too quick to yell and scream about problems because they think that fixes the problem. It doesn't, all it does is make the person you yell at, lose respect for you.