Even top-quality air mattresses leak a little air from time to time. If yours deflates a bit more than you feel it should, the problem may be an issue with the cap that keeps air in or the gasket around the air-release valve. Otherwise, the mattress may have a leak.
Check the Cap
While air mattresses vary from one to another, they all feature some type of cap to keep the air inside. Push or twist the cap to ensure it is seated properly. Remove the cap entirely, if necessary, to reseat it. Add more air before replacing the cap, if necessary. If the cap is missing or seems damaged, purchase a replacement cap from a retailer that sells air mattresses or camping supplies. In some cases, the mattress manufacturer's website may also offer replacement caps.
Test the Gasket
Some deluxe air mattresses or air beds feature a gasket inside the cap or cover of the air-release valve. Remove the gasket, soak it in warm water for 10 minutes or so, then pat it dry. Once you are certain the gasket is completely dry, place it back inside the cap, aligning it the same way it was before you removed it. Wait 30 minutes, then close the valve and re-inflate the mattress according to manufacturer's instructions.
Checking for Other Leaks
If the mattress deflates significantly while you use it and the problem isn't in the cap or gasket, the mattress itself may be leaking. Find an inconspicuous leak by filling the mattress full or nearly full with air. Wet your lips or your hand and work your way around the mattress with your ear and hand near the mattress. You should hear a small leak or feel the cool air on your wet lips or hand. Wetting your lips may sound silly, but your lips are sensitive to cool air, so they can be a big help in finding a minor leak.
Patch the Leak
Many quality air mattresses include a patch kit with purchase. Patching typically involves spreading glue around the damaged area, then applying the patch, cut slightly larger than the hole, over the area, smoothing it in place. If the leak is due to a ripped seam, you may not be able to repair the problem with a patch kit. In a pinch, smooth a piece of duct tape over the damaged area, ideally while the mattress is inflated. You can also try patching a seam with a commercial seam sealer, allowing it to dry according to the manufacturer's instructions.