Unable to Enable Crossfeed on Decoupler

[NubNibble]

I'm having an issue with a decoupler in Kerbal Space Program 1.12.5.3190 (modded) where I'm unable to enable crossfeed for resources that are behind the decoupler. I'm able to transfer resources across a docking port to a craft on the other side of the decoupler, but not from one part of my craft to another through the decoupler itself.

When I right-click on the decoupler, I can see the "Enable Crossfeed" button, but clicking on it doesn't do anything. There's no change in state, and the button doesn't switch to "Disable Crossfeed" either.

I've checked that both parts on either side of the decoupler have fuel tanks that are not empty, and that the decoupler is properly attached. I've also tried restarting the game and reloading my save, but the issue persists.

Has anyone else encountered this issue, or does anyone have any suggestions on how to fix it? Any help would be greatly appreciated!

[jimmymcgoochie]

If you’re in science or career mode, fuel crossfeed across decouplers is unlocked in the tech tree by unlocking fuel lines. A screenshot of the problem vessel would help and logs too: