Hi Patrick,
a barlow will change the focal ratio of your scope, thus your focus point will change. Try your barlow out on a bright object like a star or planet, but use the highest mm eyepiece you have (25mm?). If the image does not come into focus in one direction, turn your focus knob in the opposite direction. If this doen't work, I am at a loss for why. Ask these folks on the ETX discussion board, they may have some experience with this issue with ETXs:
http://venus.beseen.com/boardroom/d/25291/
Richard Snow
