On the wow which is not casting properly, is it the straight heal spell or a macro?

I know a lot of the pally teams go with a stopcasting in their Flash of Light/Exorcism macro, to take advantage of the Art of War procs.
So the key might be:
/cast Flash of Light
/stopcasting
Which gets you a start casting, and an instant stop casting if you lack the proc, so its not an instant cast.



If that's not the case, possibly check the FTL modifiers for the toon who is not casting.
When you click "3", it is actually doing this:
a) FTL Assist Me (for the current window)
b) Send 3 to all windows.

So the FTL Assist Me hotkey, which will be a key like F11 or F12 (or whatever) will be just the key without modifiers.
It sends that key, plus the modifiers for the active toon, to all windows so they know who to assist.
Then it sends the trigger key, in this case "3".
Check that the key being sent for FTL Assist Me, is not keybound on any of your toons both without modifiers and with any of the FTL modifiers you have selected for your team.
You can check which FTL modifiers you have set, under Character Sets and then click on the team in question.. from there, click on the 1 or 2 (under Slots) and view the options for that toon.