Air expands and contracts as it gets hotter and colder. If you add air to a mattress on a hot day, and it cools off, the air pressure will decline, making it look flat. If you inflate it on a cold day, the exact opposite will happen. It will expand and be over inflated.
To get the right amount of air in an airbed is a bit about personal preference but I sit on mine when I have nearly got it all blown up and if my bottom hits the ground, it needs more air. If it is a cold night, I always fill it till it is very firm.
Always remember to place a rug under your airbed when camping. Often they can make you feel colder than you should, simply because the air in the mattress is so cold. A rug underneath will reduce the cold from the ground rising up into your airbed.
To double check if it really does have a leak:
- Fill the bed up with air.
- Fill the bathtub up with water.
- Completely submerge the bed in the water.
- If you see bubbles then you have a leak.
- It could be a simple fix like a drop of silicone around the valve or it may be time for an upgrade.