i've experienced this on my fc660c and rf87u.
I did lot of searching on reddit, someone suggested to take some keyboard lube and lube the spring/dome inside.
What was happening in my case, was that the spring was getting 'stuck' on the dome. It kinda felt/sounded like removing scotch tape. hard to describe, but that's the closest analogy that came to my mind.
i lubed up the spring/inside of the dome, reassembled, all good.