Extreme novice typer here, working on second typewriter repair.

I recently bought an 1921 Oliver No. 9 from eBay. It was damaged during shipment and probably landed on its right tower as the tower was shorter and a bit bent. The machine also had a broken mainspring (now repaired) and a missing draw cord (now re-attached). The escapement mechanism was completely rusted/corroded and frozen. That is now moving.

The remaining problem is that the carriage does not advance while typing or using the space bar. Striking the space bar and keys produces a "stuck" sound and sensation.

However, if the ribbon vibrator is manually moved back and forth while typing, the carriage will advance. See video below.  There is an audible click as I manually move the ribbon vibrator back towards the front of the machine or when I push up on the universal bar from under the machine.

Here is a YouTube video I made documenting the problem:



Here is a second video showing the underside of the machine:



I am wondering what the source of the sticking click might be.  I have cleaned the ribbon vibrator and escapement thoroughly with denatured alcohol and PB Blaster and Evapo-Rust. The gears and starwheel seem to be spinning cleanly. Where should I look for dirt and / or mechanical obstruction?

Of note: a connector of some sort from the Tab lever is missing under the machine. While I don't think it's related to my sticking ribbon vibrator problem, it may be relevant.