If you have the config set up right, and are in test mode on Alphapix, and you are getting nothing I'm just guessing wiring issue especially since you tried another controller
For power injection coming in at the end - you will NOT push data back from the end to beginning.
At the point where the next pixel isn't working - if it is a connection point where you joined 2 strings together maybe, make sure the pixel is set up for data to flow the right direction. Pixels are set up for data to flow in only 1 direction.
If you have electrical meter - you take off your power injection off the end and see if you are getting any DC voltage at there (will be lower than 12V obviously).