The tire dust cap connector could come off sometimes. If you didn’t tighten it enough, it might come off on its own. So, it’s important to put the cap on tight. There’s also a chance that it could pop off if it gets hit. If you scrape your car against the curb, you might also lose the stem cap. It can also damage the stem itself, which could cause air to leak out. Most of the time, though, the tire valve caps come off because they are lost. It’s easy to lose the cap when you take it off to fill the tire. When you fill the tires, put the valve caps somewhere safe so you can put them back on later.