If your hubcaps keep falling off, it might be time to check the condition of your tires. If your tires are old and worn out, they might not have the same grip as they used to. This can cause the snap on hubcaps to come loose. You can try replacing the tires or adding some tire adhesive to see if that helps.
Another possibility is that the snap on hubcaps aren't properly snapped on. Make sure that you're snapping them on securely and evenly all the way around. It might help to practice a few times before putting them on your car for real.
If you're still having trouble, there are special tools that you can buy which will help you get a tighter grip on the hubcaps. These can be found at most auto parts stores. With a little bit of effort, you should be able to get your snap on hubcaps to stay put!