I know Westside says very often that the best way to train the deadlift is to... not train the deadlift. Although that doesn't really apply in this case, since they simply train tons of assistance exercises.
I'm expecting you haven't been training their way... here's what it may be:
The training you've been doing for your deadlift is in fact de-training you. Perhaps you need to work on your form, you need to be more aggressive in upping the weight when you can handle it, or maybe you're training the deadlift too often, before your CNS recovers fully (so a full rest you're able to do more), etc.
Maybe these reasons don't stick with you, but I can't think of anything else. I'd be happy with that if I were you, 315 is a great pull at your weight!