I had this problem with my 2 Vectrex systems back when I had them and I never could figure out as to what was causing it. I had one Vectrex that was made in Japan IIRC and the other was made in Honk Kong or Taiwan. They were made in different countries but appeared identical in every way. On one system, I don't remember which it was, Clean Sweep would not display the walls of the maze. It was NOT a brightness setting issue. I tried the system with 2 different multicarts and it did it with both carts. Then I bought an original boxed Clean Sweep, thinking maybe the ROM was copied wrong to the multicart, even though it worked correctly on my other system. It did the same thing with the original Clean Sweep cart. Every other game displayed fine, no problems, text was fine too. Has anyone ever experienced an incompatibility like this?