Thank you! I've also seen people use key oil (like for clarinets)...I've ordered some of that to try as well.

HI!
I often times found out that on most Zithers, when you replace your string, you don't turn the tuning pig often enough. So what oftentimes happens is that (always the C-String) the tuning pig doesn't have any space anymore to be turned.

Maybe try to get your strings off, turn your tuning pig 2 or 3 rounds more and then put your strings back on again.