Throwaway lines like "always aim to keep the bellows as tight as possible" are too specific and not very helpful when you don't know the style of the player you're giving advice to - even the tune they're trying to learn has an impact on whether that is true or not. I would add that, as a player who uses the full extent of the bellows, I can assure you that there are reasons to use the whole extent sometimes, especially if you play using chords on both hands and it's not just for show!
Air control is a tricky beast at the best of times and the only real advice that works is to make sure your bellows are in the right position for the next passage of notes. This depends on so many factors (chosen fingering, number of buttons pressed, required volume, the tune being played, etc.) that it would be a surprise indeed if any two players had the same approach. It has always been a hit and miss affair which you can only really resolve by repeated practice - at least to start with. I often liken air-management on the melodeon to someone playing snooker ... while it may be possible to pot a ball or two, you can only really win a game by thinking about the position the white ends up in for the next shot. Similarly you can only really play a tune well if you work out where your bellows need to be for the next note. The very best players of both can do this almost instinctively in any situation, but that only comes after hours and hours of practice.
This may not be pertinent to the original post though ... we haven't established whether there is an air leak somewhere, or whether the bellows are incredibly stiff - both of which could cause the symptoms described as well as struggling with technique.