I joined this form because I wanted to reply to you and provide my insight which I hope is helpful.
I agree 100% with this. First, I'm not sure how often you are doing heavy deads, but I'd limit their frequency. Deadlifts are extremely taxing on the CNS and it's not a lift where doing them more often is going to help it go up. As Ramo said, you need to address your weaknesses in order to increase the lift.
It's my opinion that many guys could benefit greatly by doing SLDL's off of boxes with moderate weight along with hyper-extensions (back raises) and tons of abdominal work. If you pull sumo, zercher squats off of pins help greatly. I also like to do a ton of concentric squats out of a power rack...take your deadlift stance and look at where your back lines up on the rack and set the pins there...load up the weight and squat it up. I've heard a similar exercise called "dead benching" so I guess this would be "dead squatting" but really it's just the eccentric portion of the squat without the eccentric and without stretch reflex.
Again, going back to what Roma said...you need to think of the muscles involved with the lift as a chain, and the weakest link in the chain will hold you back. If you continue to try to pull the same load, the same links are going to break. You need to use other exercises to strengthen your weak areas to bring the lift up. A good majority of beginners are very weak off of the floor, which is why I recommend making friends with SLDL's and boxes, it will build tremendous starting strength, to the point where you'll get to heavy weights that you blast off of the floor (at your weakest leverage) and you will be unable to finish...that's where pin pulls and other stuff comes in...for now, SLDL's and boxes, hypers, lots of abs and continue with your squatting regime...goodmornings are also a great exercise if you are comfortable doing them.
Good luck and I hope this was somewhat helpful.