It might be that someone has adjusted the carriage shift too "light" and the threaded adjuster is sticking up and stopping the movement of the carriage when it is resting in its normal position. Move the carriage by hand to the left or the right and look along the back wall of the case for a spring that hooks into a threaded rod and has a lock nut on the top. There's one near each corner.
   Or it could be something else entirely. Get a flashlight and sight along the gap between carriage and the rest and work the carriage back and forth with shift on and shift off. Good luck. PS SM4's are notorious for the case-to-chassis rubber grommets getting squished over time and that lets the sheet metal case "rise up" and catch the carriage.  There are lots of mentions of this problem in this forum if you do a search.

Thanks for that post, Beak. I checked both my SM4 and SM7 and found that the carriage lock  wasn't working on either of them. I played with the SM4 awhile and oiled it up and found a way to use the lock. (Truth is, I never used it before. I didn't know it even had one)
   The SM7 lock doesn't seem to be hooked up to the wire anymore. It doesn't move at all when I flip the lever. I'll have to get in there and figure out how to reattach things.
   Thanks for the tip.

it might be the rubber spacer washers underneath, between the frame and case have perished, so the outer case sits too high and catches when typing in lower case, look for tell tale signs on top of the outer case under the carriage, there might be scrape marks. I generally replace these with Tap (faucet) washers.