I just saw this on Discord:

"There's been a change to the API server that causes Neon to not show your unavailable GAS. If you make a transaction and send all your NEO to yourself your unavailable GAS will become available again and show up in Neon for claiming. This should be fixed in the next release of Neon."

I'm in no hurry to collect GAS, so I'll just wait for a Neon wallet update