Any fired case, brass or whatever, is going to be a tight fit even in the chamber from which it was originally fired. After being fully seated, it might well have to be forced out especially from the chamber of another rifle. That unfired cartridges slide in and out effortlessly is also the norm.
In a BA, benchrest and target shooters can get away with neck sizing cases for a couple of reasons: the cartridges are seldom required to feed from a magazine, the primary extraction system has a great deal of mechanical advantage in terms of both leverage and camming action compared with most other action types, and the lock-up of the breech is extremely consistent and rigid.
Many reloading equipment manufacturers recommend the use of special "small-base" dies for ammunition that is to be used in lever, pump, or semi-auto actions to ensure that case dimensions are returned to as near SAAMI minimum spec as possible to head off feeding and extraction problems, even if the ammo is to be used in the same rifle it was originally fired from.
The curious part of your problem to me is that so much force is required to remove a fired case. All four of my own SKSes have chromed chambers and barrels. Even after a couple of hundred rounds without cleaning I can chamber and extract a fired case without having to use anything other than my hand on the charging handle. That's the main reason I suspect that your chamber isn't as smooth it needs to be for reliable functioning anymore. Something is making it grab the walls of that case more than it should.
Even if both of the examples in question have chambers at the lowest end of the tolerance spread (statisically very unlikely, even given consecutive serial numbers) that still shouldn't happen if the finish texture of the chambers is anywhere near spec. Yugo ammo is indeed brass cased for the most part, but I can't imagine that a Warsaw Pact weapon wouldn't be designed and manufactured to work just as well with the mild steel cases found on the vast majority of 7.62x39 ammo. It just wouldn't make sense, especially in light of the Cold-War-Turns-Hot scenarios of the time.
IIRC, the grenade launching feature is basically a cut-off of the gas system. If that port is closed, either by the switch or obstruction from fouling, it would render the semi-auto functioning null. If inspection with a strong light in the bore doesn't result in seeing light through the gas port, check to make sure that moving the switch indeed opens the port. If it does, run a reaming tool through the port to make sure that it isn't occluded with fouling. If your issue cleaning kit doesn't have one in it already, they're cheap and readily available from many sources.
It is important that all of the operating parts in the gas system move freely. If your primary piston doesn't exhibit full travel without binding when both the cylinder tube itself and the smaller rod tube are clean, find out why and correct it. To check the secondary op rod and its spring, move the lever of the handguard latch on the rear sight housing up until the rod end is clear. It should come out on its own, and with some force, so be ready to catch it. If it doesn't, you've likely found the biggest part of the problem. Scrub out that tunnel with solvent and/or penetrating oil. Get all of the crud or corrosion off the rod. If the spring is corroded, or just feels "weak", replace it. When lightly lubed and reassembled, it should move freely both ways when the head of the rod is pushed with a punch or the piston rod.
It's tough to assess just how rough the far end of your chamber and the neck area really are visually without bore scope, unless things are really pretty close to FUBAR. You might try this trick, though: Take a piece of small-gauge spring wire about five inches or so long and make a 90 degree bend on one end about 1/16" long. Sharpen the end of that to a fine point. Insert it into the chamber as close to the origin of the rifling as possible. Pull it slowly back and forth, paying particular attention to any areas that look even slightly darker than the rest. You should be able to get a reasonable idea of the comparative smoothness of the surface from the feel. Any areas that show marked resistance to a light pull, or "chattering" will tell the tale. it should slide relatively smoothly and quite consistently. If it doesn't, your chamber is damaged, and you should take it to a professional for further evaluation and/or repair.
While cleaning the chamber up with 3-4X steel wool wrapped around an old cleaning brush and soaked in a good penetrating oil like "Liquid Wrench", spun either manually or with an electric drill at very low speed, might help some I would strongly advice against any more agressive polishing agents or methods. Too easy to remove metal, and too hard to do it precisely and evenly. The potential consequences aren't worth risking.
I've had at least one SKS that took three brushes to get clean. Trust us on this - just do it.
