My new tank sender unit isn't moving.

The tank senders unit on the outside of the tank works by picking up a moving magnet in the level indicator valve fitted in the tank.  The level indicator in the tank has a float fitted so as the liquid level goes up and down the magnet rotates, and then it rotates the sender unit you fitted to the tank.  This works by using the same principle as putting 2 magnets above and below a table work, so when you move the bottom magnet under the table the top magnet moves the same, but it will only do this if the 2 magnets are allowed to pass each other first and thus link magnetically.

If your tank sender unit doesn't move then 99.9% of the times it simply because the magnet in the tank hasn't t got the sender unit agent so at some point this has to pick up the magnet in the tanks float valve unit for it to start reading correctly ( start moving ), so depending where the sender unit has been left on assembly and where the float is in the tank, they might not align until the float level valve next goes past the gauge which could be when going from full to empty or empty to full.  Once the magnets are aligned they will stay aligned.

If your unsure of a problem then simply remove the 2 small self tappers screws you fitted the sender unit into the tank with, put a magnet or metal screwdriver behind the sender unit and move it so the sender unit rotates.  This should move the LED display reading ( if its wired up correctly ). If you have a wiring issue then please check out the wiring test section here using a Ohm meter to take reading and check the sender, the wiring and the indicator.