20" is usually well over enough to string a cross/main.
Make sure you tied off one side of the mains first. With the string that you pulled through the grommet ( the side that couldn't reach the tensioner ) clamp that string on either side of the clamp. Then get scrap string and clamp it on the remaining side. Then pull tension like usual.
Did you make sure you had an even 10 ft on each side of the mains?
Did you use a stationary object and cut the string in half?
Racquet : Prince Youtek 6.1 Tour 531g 17x19
Strings : Steel Wire ( 21G ) <- Prototype