yeah, you have a stuck link under the keyboard. Flip it over and pull off the silver tray under the keys, then hit those linkages with PB or LPS1 and then find that stuck linkage and work it back & forth till it frees up. Machine-gunning in a SCM electric pretty much always is one of those links being sticky. (:
I was just thinking that Jittery Apostrophe would be a great name for a typewriter blog.
Ted is probably correct in his diagnosis but you can rotate the power roller by hand and observe the actuator working. If it is sluggish just wash it it out and a spot of light oil should do the trick. Look at all of them while you are there. Also, make sure that the type-bar action is free.