It doesnt need to be that clean just make sure its not sticking and oil the cable with oil. Not WD40 as thats not an oil, incase thats the issue.
Failing that a new inner cable might be worth a try if you feel its got a kink in it. Make sure your cable terminals are present and intact. Sometimes the hole in the end becomes enlarged and the cable tension is lost.
I now fit brass spoke washers behind the cable terminators now to avoid this common issue.