This is typically caused by having key bindings configured within the game, overriding the /assist {FTL} macro ISBoxer generated. It's configured to use <slot-specific Modifier> + Backspace, and your slot-specific Modifiers are the defaults: (1) Left Shift, (2) Right Shift, (3) Left Ctrl, (4) Left Alt, and (5) Ctrl+Shift.

The ability to assist slots 1 and 2 should actually be the same since WoW doesn't let you key bind left and right shift+Backspace independently, so I'm curious as to why 5 would assist 1 but not 2.

Anyway, the expected fix is to do the following for each instance that fails to assist another:
1. Turn off Key Maps (Shift+Alt+M)
2. Open the in-game Key Bindings screen
3. Find something that isn't bound at all (an empty box) and click it
4. Set it to Shift+Backspace and observe at the bottom what it says is now un-bound or if it was not previously bound
5. Repeat 3-4 for other key combinations Ctrl+Backspace, Alt+Backspace, Ctrl+Shift+Backspace (depending on who they won't assist, but to be safe you can do all 4 of the combos -- left and right cannot be bound separately)
6. Unbind the previously-not-set key binding you were borrowing