This issue is really hard to explain in words, so if you need an video, I can provide one. I use a OpenBlocks Block Placer to place Cake for Kekimurus', constantly being pulsed by redstone via an Extra Utilities Redstone Clock. It seems whenever I try to use the multiblock preview, and the Block Placer tries to place a block, it binds the preview to the coordinates of the Block Placer, preventing the player from really using it. I haven't tested if this works with any other block placing method.