You were probably tired. According to a squat rep calculator that I use, you should be able to get 6 reps at 385 if you can get 355 for 9 reps. Again, you were probably tired.
Do you have a spotter? Do you use a belt? Why can't you go below or equal parallel? I am thinking that it has to be a psycological thing for people not reaching parallel, something about fear of getting stuck in the hole. That's why I ask about the spotter. As far as the belt, I'm curious because I never use one and the few times I have, I never felt like I benefited from one or felt like I needed one for safety.
Keep it up!