I couldn’t find a video but I did read that the C string on a viola tends to stretch when played and you have to use more pressure when playing the C string. 

I don’t know what issue you have, so couldn’t refine a search. 

One article said the put the bow in the C string. Draw it up to the tip. Check the wrist and elbow for proper positioning, which is really no help if we don’t know how it should be positioned for that C string. Then go to the frog anf check again. The elbow should go no higher of lower than it is in either position when doing the C string. Pretty much not very good info.

On the bright spot, apparently, the C string is not used much. But, I think they were talking in an orchestra because they said that, and followed up with notes on the viola C string are usually covered by a cello. 

I am going to get my viola down and check out that C.

I have a 15 1/2”. My 16” was a little hard in my arms so I traded it in. I love the 15 1/2”. Maybe scaling the size down to a 15 1/2” will help?