How does 1ML measure the availability ranking of nodes? If there is an 1ML Node Rank ranking explanation anywhere ? 
My node is online 99.9% but "Availability" dropped down from 700 to 4000 in last 3 months. :( 
I guess this is somehow associated with the number of channels ?
Looks like the problem is found.
I didn`t forwarded 9735 port on router, as LND and my router support UPnP.
But looks like UPnP works not as intended, so sometimes host was accessible on 9735 port, and most of the time not :(.

Now I forwarded 9735 port to my Raspberry Node local address , so  Availability on 1ML not dropping down.