Philips 43pp92 blue will not converge

I have a 3 year old 43pp92 philips rear projection 43" HDTV, just the other day the convergence went out of whack, I was able to converge the red and green pin cushions, however when i tried to converge blue, it lets me converge the ones closer to the middle, but Top right corner and Bottom left corners, the little crosses will NOT adjust up or down, just left and right. I am usually a 'do it yourself' person and not afraid on taking a chance destroying this TV and I'd like to fix it myself, any suggestions on what the problem with is, and/or how to go about fixing it