A couple things IMO. First off, sounds like it could be somewhat psychological as you get closer to #400.
Have you tried micro-loading? Very small increments like #1/2-#1 pound. Doesn't sound like much, but it can add up over a month.
Pin presses at various heights off chest?
Adding accommodation, say light chains, to increase load moving to lockout?
Dropping some weight and doing some higher rep work/volume for a few weeks then slowly over time working the loads back up?
(Depending on how long you have been chasing heavy weight, (one's CNS gets tapped out) maybe you just need a little break with some lighter work to get some momentum going again!?)