Spartan Rage not working sometimes

[monitor]

Why sometimes pressing left shift with middle mouse button does not activate Spartan Rage?

[Monkey Man]

If you have previously used either of the two buttons the game will think you want just that button to be used.

For example:
If you do a sprint attack then press middle click (while holding shift) the game won't recognise the input.

If it's just at random then i'm not sure, i do have an issue where kratos will just stop walking even tho i'm holding down the W key.