For ISP Elite MK1, 2ISP and ISP Master: check this documentation:
For all units:
Check that you connected your IR eye at the back of the unit on the “IN” IR bottom port right.
Check your IR receiver pinout and invert it if necessary.
Check through a camera lens that the IR emitter is working on the Remote. Change the batteries if needed.
Check that the IR receiver is receiving the IR signal (either with a flashing LED or through a camera lens).