Open the coupler and spray a little graphite powder in it. Open and close it a few times. You could also use PTFE. Since it is in a liquid carrier it might penetrate more deeply. The carrier will evaporate leaving the PTFE behind. Don’t get it on poly carbonate or polystyrene plastic otherwise, it is safe.
Somewhere in my belongings I have a small bottle of PTFE in powder form, no carrier. Might be something from the HO or N crowd. An internet search of "PTFE powder comes up with all sorts of hits, and its probably on Amazon and fleabay.
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