I'm partly guessing here, but I think that when the market updates, it just randomly picks planes from sales & storage. In JA there are currently 800 planes on the market, and something over 5500 in storage. So call it 5000 to select 50 planes from. It's basically a 1% chance of a particular plane appearing in an update. If a fleet group only has 3 planes in storage, there's a little under 3% chance that fleet appears in an update, roughly a 50-50 chance of seeing one plane of that fleet somewhere in the next 23 updates.
Instead of selecting the planes completely at random, would it be possible for fleet groups that aren't currently on the market, but are in storage, to have an increased (say 20 or 25%) chance of being part of an update? Right now, I count 36 out of 40 fleets actually having planes visible on the UM now. So it'll make a minimal difference to what goes into an update, just make it more likely that all fleet groups the brokers have in storage are actually on the market. If a plane doesn't sell, then there's no bonus to have the brokers put more out there. If it does go quickly, then there's more chance of another one being added. Will be some encouragement for people to use different fleets, currently at the start of JA there's no point picking a fleet with only 20 planes in storage, as you might only see 2 appear in the first 6 months, better to stick to some of the 5000 DC-6s. Make this change, and you can pick those smaller stored fleets, knowing the brokers will actually keep sending them out there as long as they sell.
Plus, I won't spend 12 months refreshing the UM without finding a single tupolev, when I'd be happy to take all 3 in storage right this second.