Finally moved Neo from Bittrex, to be able to generate Gas

I'm using neo-gui wallet. Does anyone know what the logic is behind Available and Unavailable gas in the "NeoGas Claim" option? Why is some gas Unavailable? Is there some way to make it available?
Thanks,
Jb
Figured it out - have to transfer NEO to be able to claim gas (to make it available). You can just transfer to same address and this makes it available so you can claim.