How to reuse a slot?

I’ve purchased many more slots than I have aircraft in each category but still can’t make any of my aircraft depart. No available destinations. The destinations slots show that an aircraft that is now waiting to depart is “At Airport”. Multiply that by 30 small and 20 medium.

The only way out of gridlock I’ve found is to kill and relaunch. That suggests to me that this is a bug. Anyone found a way to make slots reusable without restarting?

Sounds like you are having the same issue as Icegamer1705 in his post from earlier today.
Does his post describe your problem ?

No. My issue is with departures for my aircraft. The departure slots never clear when an aircraft returns. I have more slots than aircraft owned but unless I buy more or restart the game I end up in gridlock with no aircraft able to depart and the arrivals queue full and unable to land.