[Closed] SRAM X3 Trigger Shifter Problem

On my sons bike the SRAM X3 trigger shifter for the rear derailleur is not working correctly. I have the derailleur set up in hig gear fine and connected the cable. Changing down to 6th gear is fine but then trying to change to 5th the shifter clicks but the cable does not move, the derailleur does not move and the gear indicator on the shifter does not move. If I detach the cable I can click up and down all the gear indicators on the shifter. Really stuck for what the problem is.

Grateful for any suggestions before I pull my hair out!!!!

Thanks