I had similar issue FO was repeating command but nothing changed.
In my case it was A320 a/c and I asked SET HEADING 320.
FO replied SETTING HEADING 320, I could even heard clicking heading bug, but nothing changed.
The reason was FCU mode was in the TRK/FPA, so my command was not correct. FO will be confused if A/C is in TRK mode, but I ask him to set HDG. Once I asked him to SET AUTOPILOT TRACK 320 he responded and set the track I wanted.
So, sometimes we should just make sure we give correct orders which are in line with current A/P mode.