While an 8-deck game requires a large bet spread to be profitable, the Hi-Lo will still provide an advantage. The concept is well-established and not in doubt. If you're seeing results that dispute that, there's a problem with the simulation.
It takes a lot of hands to eliminate the luck factor and insure that you're seeing valid numbers. Perhaps you simmed too few hands to accomplish that, or perhaps there is a flaw in the software. What software is this?