Hubcaps serve a purely aesthetic purpose and, depending on the type of vehicle you have and how much its appearance matters to you, you might spend anything from a few pounds to several hundred pounds on a replacement set. Either way, it is annoying to lose one and, despite improvements in design over the years intended to keep hubcaps from falling off, it is still a common problem. Losing one hubcap can mean having to buy a whole new set if you can't find one to match the rest, making it even more frustrating when you find one missing.
Make sure that the clips holding your hubcap on are positioned correctly on the wheel. Most hubcaps come with some kind of retention device, such as a plastic or steel clip (depending on what the hubcap is made of) that fastens onto the wheel to keep the hubcap in place. Sometimes a bolt-on fastener is used to keep the hubcap in place, or a plastic washer attached to a lug nut holds it on. Whatever retention device your hubcap uses, make sure it is fitted correctly -- in the proper position on the wheel and fully clipped into place if they are the clip-on type or, if they are the bolt-on type, make sure the bolt has not come loose.
Clean any lubrication off the wheel around the area where the hubcap fits. If some lubricant has been put on the wheel to help the tyre slide onto the rim more easily -- for example, after a puncture repair -- this could be the reason your hubcap does not stay on.
Wrap clip-on retainers three or four times around with plastic electrical tape. This may be enough to get them to stay in place. First, wrap every other clip this way and try the hubcap to see how well it stays on. If this is enough to hold it, don't wrap the rest of the clips as it may add too much bulk for a tight fit. If the hubcap still seems a little loose, however, wrap all of the clips.
Sand back the paint on steel wheels that are painted -- sand them where the clip fits on. Some coarse 80-grit sandpaper will make the paint rough and help the clips stay on. Don't sand the wheel back to bare metal, though, as the clips will not stay on so well.
Attach the hubcap with a couple of cable ties if all else has failed. This is a tactic that police departments use to solve the problem on some of their vehicles. It might not be the prettiest solution, but if you can hide most of the cable tie, it will look all right and will definitely keep your hubcap on. Trim off the excess tie with scissors. Some car parts suppliers now sell kits with silver ties that will blend in with your silver or grey hubcap. They sell the ties with clippers to cut the ties off when the tyre needs to be removed and spare ties to use when replacing the hubcap.