"Secondly, why do you not just check the shift cable?"

Agree totally. Shift cables are relatively inexpensive and easy to fit. Properly installing high-quality die-drawn stainless inner wires and lined outer housings can hardly fail to improve your shifting. If you still have problems you will at least know that the cables are not contributing to it.

Some details of your bike and drivetrain are needed for us to be of any real assistance.