You shouldn't have to "push" a lifter into its bore. After the initial line up, it should slide into its bore under its own weight. Obviously, this would be checked during part fitment/mock up. Also during this time, you could have installed all the lifters and rotated the cam with a speed handle and visually inspect for rotation. I suspect something with the lifter bore. It doesn't take much to make them NOT rotate. Have you tried swapping some of the other lifters around in that bore?