Hello!
If you were like me and none of the above resolved your isssue…
The above did help lead me to my issue. In my case I was using the default 1st person character blueprint which I had duplicated and altered for my needs.
In doing so it was still using the default characters IMC_Default that is found in the inputs folder within the character. Note that this only comes loaded with the basic inputs ( jump, move, look, etc)
Either making a new IMC with new action inputs set , and then ‘Add Mapping Context’ with your new inputs.
or simply add your desired Action Inputs and set them to the IMC_default accordingly.
I hope this helps 
-DxE