My boys and I just got a Monoprice Select Plus 3d printer for Christmas and I'm having trouble figuring out to how to home the printer. Something doesn't seem right and I'm not sure if it is something I am doing or if there is a hardware issue.

What I'm doing and what I'm seeing:

I turn the printer off and I adjust the horizontal bars to the same height on both sides from the frame.... this printer does not have synced z-axis motors, so I do this first to the the print head somewhat level to the build plate. I then bring the print head all the way down and adjust the build plate to a piece of paper thickness from the nozzle at all 4 corners. At this point I don't see how I am doing anything wrong. But it seems like the printer just isn't doing what it is supposed to once I turn it on.

When I turn the printer on, and choose the Home option, the print head appears to be properly finding the home position in the X and Y axis, but the Z axis it always lifts up about 1/4 - 1/2 inch. and it does that EVERY time the home option is chosen from the menu. If you push home enough times the print head will hit its max height. It never seems to move DOWN and find the real home position.

Just to see if it really knew where home was I tried to print something from the provided SD card and it just prints at the same height it moved to which right after my manual calibration is about 1/2" above the build plate. How do I tell the thing where its 0 point is on the Z axis?

What am I doing wrong? Do I have a sensor not connected somewhere? Is there a problem with my unit?