If the valve body is a reverse pattern manual shift unit it will never shift as long as the selector is in D because that is 1st gear. You'd have to manually shift it to 2nd and then low 1 would be high gear. If the valve body is not this type it should have a kickdown linkage. I'd unhook it momentarily to see if that's the problem. If that's not it I'd take it back to the reputable person and have them check for a sticking governor or valve in the valve body.