I suggest trying a Radio Shack FM trap & insert it between the VU-190 & 1 TV (no amp) to see if channel 6 appears. You can always return the trap if it doesn't help.
Could be man-made interference that is preventing channel 6 from being seen. Overhead power lines or various electrical devices inside the house can easily smash VHF-Lo signals that are weak. Move the antenna around as much as possible & re-try 6. Have you tried manually entering "6" or "6.1" to avoid having to re-scan?
Not sure if it is worth buying a higher gain antenna if you don't see channel 6 at all with the VU-190. If it is possible, you should be seeing something. If you really want to try a higher gain antenna, the
Winegard YA-6260 has about twice the gain of the VU-190 on channel 6.