When did Shimano UN-7X series turn to using "plastic" cups? (this is sarcasm for those who do not recognize it)

There is often thread lock gunk on the cups, so it gets harder and harder to turn the cup as it is inserted deeper. Dirt on the threads on the BB shell or the cup will make it harder to turn, first try cleaning. Also, is the BB new? sh1t on the inside of the cup or on the outside of the cartridge shell will seize things up. Grease the lead in on the cup, there may be just enought misalignment that the cartridge shell is hitting the edge of the cup instead of sliding in. loosen the drive side a few turns (so that the BB will wiggle a little) and try installing the non-drive side cup. If you are replacing the BB on your bike, there is less chance that the BB shell threads need chasing. If you are cross threaded, it will sieze up in less than 3 turns, inspect the ALUMINUM cups andthe BB shell and seek professional help if they are crossthread.