Almost certainly a media drive speed issue. You need to have fast external media drives to play multiple streams of HD. Firewire 800 7200 rpm drives can pretty much only manage a single stream maybe 2 at the most depending on format.
Once you start needing 4 or 5 streams of HD you have to look to raids and other interfaces like esata or fibre or thunderbolt etc etc
best wishes
Neil
http://www.patience.tv