Check for crud around the firing pin, if you haven’t already. One little flake of gunk could be enough to cause the malfunction (insufficient force to ignite the primer). Old/excess lubricant could do it also, and in cold weather just about anything not rated for arctic use will tend to gel.
You got me to thinking. I don’t think I have ever cleaned that Unique a single time. I have probably wiped it off a few times but it is totally either chrome plated or aluminum so not much rust.
Despite the fact that it is extremely well machined, the original plastic grips were garbage. They both cracked so I made new ones from the side of a TV set which was struck by lightening.
I am not a craftsman so they don’t look great but they work perfectly.
I am going to take your advice and thoroughly clean the slide inside and out. The rest of the gun too.