xyz calibration problem  

xyz calibration problem

I have built this from a kit, all self-check tests passed with the exception of the filament sensor test - but it was recommended to me by others that I just unplug that as it has caused fires for some users. I'm not too worried about using the filament sensor.
When I get to the step for xyz calibration, it fails. Here is what happens:

From calibration menu, select "XYZ calibration"
"calibrating z axis"
the extruder moves back and forth a bit, then up the z-axis. (The video walkthrough at this point indicates that the z-axis will rise all the way to the top. It doesn't do this at all - it really only moves about halfway up)

It asks me "Is steel sheet on heatbed?" I answer no.
"please clean the nozzle.."
"Place a sheet of paper...." I put a sheet of paper underneath and click the nob

"Searching bed calibration point 1 of 4:"
The z-axis moves down all the way to the bed, then immediately stops with message "Calibration failed! Check the axes and run again."
The nozzle is not touching the bed - just slightly above it. The pinda probe is slightly higher than the nozzle, as per instructions.

What steps should I take here? I see a lot of messages about calibration failure... I'm not really sure where to start. I had re-done my threaded rods going through the trapezoidal nuts, as one side was stiffer than the other. Should I look at that again? Would it be a good or bad idea to grease that trapezoidal nut? How freely should it move when turning by hand? I can fairly easily move it by hand, but it does take some amount of hand strength to do so. When I move each axis independently using the motors from the settings menu, they all move freely including the z axis up and down.

I've verified that the pulleys do not slip, and the print nozzle is clean (it is as yet unused). The print nozzle does not touch the bed, but this calibration procedure isn't running very long before if errors out. Do I need to do something with the PINDA probe? I've tried raising and lowering it a bit, with no change to my calibration results - the same thing happens every time. The autohome procedure runs through fine - the axes all seem to move freely during that procedure.

I feel like I"m one step forward and two steps backward every day with this build... I'm trying to build this printer for my robotics classroom, I'm a complete newbie to this and received the kit as a gift for our robotics team. I'd love to be able to finish this before holiday break - this is turning out to be a great lesson for my students in frustration, problem solving, and persistence haha. It's hard for me to find a chunk of time to sit down and figure this out, so this build has been going on for what feels like endless weeks.
I'm glad I've made it to this point.... but now I'm stuck on calibration. Any advice is much appreciated! What are the most common culprits for this error?
Edit - just to add that I've also tried a factory reset with the printer, and the same thing happens when I try XYZ calibration.