Good progress there. The bowing is almost certainly due to the stress relief when the first side was ground. To be frank, I don't know why you bothered to get it ground, since in use you have to put packing under the plate and a felt "blanket" on top, which will take up any small unevenness in the platen. I think the springs are in the wrong place - really you want them to take the weight of the top roller to make it easier to get the plate/blanket sandwich under the roller, then use the screws to apply compression. You have allowed enough space for the plate/blanket?