AEG - Air Seal
Nozzle - Hop-up
Another place to test is the seal between the nozzle and the hop up. This is hard to test, and also hard to fix. But making a good connection is the key. The Air nozzle should make good contact with the lips of the hop-up rubber. A loose fit will let air out.
If you have space between the nozzle and rubber what you can do is to shim the hop-up closer to the gearbox. This is a problem on allot of older and cheap guns.
The most common problem on guns (based on what i have seen) is the hop-up unit housing (Mostly on plastic ones) being so wide that the rubber don't press on the barrel. That means that the air escape out between the rubber and barrel. This is easy to fix with some Teflon tape around the rubber. When pushing the barrel with rubber and tape into the unit it should have resistance.
The final place where air seal is an issue is inside the barrel. The problem is that there's no way to increase seal yourself. You have to replace the barrel with a tight bore barrel. Where bb's will seal better.
But remember that it's a matter of a few fps so don't go changing to tight bore before you are sure that's what you need. And also be aware that the ratio between cylinder and barrel volume does matter, so smaller diameter barrel may need shorter barrel. Use the “Volume calculator” on the Calculator page to check ratios.