Finn:
Try some Marvel-Mystery oil. Its should be safer on the rubber corner seals than carb cleaner.
Never used it in a rotary but it did the trick for us when when had sticky valves and ring in the Lycomings and Continentals.
The secret ingredient is Wintergreen Oil (Methyl Salicylate) that's both a solvent and a lubricant.
Additionally, Valvoline Restore and Protect has worked well to free up sticky oil wiper ring issues on late model Volvo engines that have a known problem with overly tight oil ring groves.
"a synthetic motor oil designed to actively clean engines, remove up
to 100% of deposits with continuous use, and prevent future buildup"
Mike in Maine