The reason for the problem is probably that you can't crank the lens close enough to the negative for a big enlargement. For an 8X10, the back focal node of the lens is only about 56mm from the film for a 50mm lens. The first question is whether the enlarger was designed for 35mm negatives. Most are. If so, then check the lend board and check for any obstructions that prevent the bellows from bringing the lens close to the negative. If all else fails, you can still make moderate sized enlargements using a 75mm lens with 35mm film.