Center serving comes in different diameters like .018, .021, and .025" . you may have to either bump up to the next diameter size for your center serving, or get a new string with more strands in it to increase the bundle diameter.

I have known some people that want to get around this and they lay an extra strand or two of string while they serve over it to increase the diameter- I don' t like that method but I suppose it may work in a pinch.