O-ring won't seal

[Animal]

... on a plastic bit subject to a few meters head of primary circuit water. Have cleaned it up but refuses to seal. Looks fine, can't remove it without trashing it & haven't found a replacement so far. PTFE tape on threads also didn't sort it. Is grease the next best move? I might have some red rubber-safe grease somewhere, if not what are the options? Whatever is used must never set.

[John Rumm]

Silicone grease might do it, if not then Fernox LS-X - it sets kind of - but not enough to stop you taking it apart later.

[Animal]

More wraps of ptfe on the thread plus plumber's grease on the o-ring and it sealed. It shoulda been easier but it's done. Cheers.