Cannot Share Screen with MBA which is behind Airport Extreme
I am missing one critical detail in your description: How are you connecting?


VPN
iCloud / Back-to-my-Mac
LAN


Meanwhile - assuming it's either 1. or 2 - these are the quick check's I would perform initially and one-at-a-time. 


Check if it works the other way around: MBA -> iMac
If you connect via VPN, try Back-to-my-Mac instead
Make sure all routers run with current firmware
Both routers should have UPnP (non-Apple) and NAT-PMP (Apple) enabled. 
Some 3rd party routers allow more fine-tuning with UPnP. While this basically opens up your network (and makes it more insecure), I would go and open up for now, just to see if this solves the problem.
(temporarily) disable your firewall on both Macs. If you are running any 3rd party firewall app (such as LittleSnitch), switch if off for now.


Glad to hear if any of this opened your door again.