I was just thinking the same thing. I think they only show Buffy's bite mark from Angel when it's important to the story. When Parker is asking about scars you see Buffy's bite mark, Angel had caused Buffy the most emotional pain at that time, she wasn't over it yet. I think the reason you never see the Master's scar is coz Buffy got over that, so there was no point bringing it back into the story line.