I have a magnetizer setup with 20 iron fences below it and none above. It is receiving full power but after one trip put it doesn't propel me all the way up again until I come back later. This has been repeated many times and the result has been the same. It seem each fence is propelling the player and taking it's own energy from the magentizer, so the fences are draining twice as much power as the magnetizer can provide.
After draining the fences from the first time going up they seem to still try because they player is nudged up a tiny bit about every second but immediately lands back on the floor.