I have the same problem. Currently in the process of rebuilding my airline, but immensely held back by the lack of slots airports are offering. I have plenty of money and aircraft, but without slots I cannot put that to use.
One of my bases is currently scale 3 with only 20 slots, meaning I can only operate a small number of routes on a lower than desired frequency. All of those 20 slots have been taken up since the first day and none have been added ever since.
My other bases are even worse, only offering 10 slots which too are all occupied since day one.