You need to focus on working your posterior chain. 505lbs is a solid number but to hit 6 or 6+ everything you do should be utilized to help your deadlift. Do you squat olympic style to help your deadlift? You should. Are you hitting GM's, RDL's, SLDL's, Glute ham raises, Deficit deads, etc into your routine. You should. Are you rotating your assisting movements every so often (i do that every 8 weeks)? You should.
You should be rotating your assisting movements in order to keep making gains but they should all have one thing in common and that is to continue to improve your psoterior chain (calf, hams, glute, and lower back).
You need to train wisely when you want to start pulling more. Also are you utilizing percentages to keep pushing yourself or are you arbitrarily coming up with those numbers to pull when you train? There is a science to pulling and increasing it.