It probably has nothing to do with the mounting block. You just have a horse that does not want to stand when mounted, a common problem.
Your horse just figured out that mounting block = being mounted. She also figured out that if she moves off, it is difficult for you to get on.
So this is what I would do. Place the mounting block in the middle of an open area (such as an arena). Make sure you have plenty of room. Then attempt to mount the horse as you normally would. If the horse won't let you mount, then either make her back (you may have to hop off the block) or circle her around the block (I would try circling Before backing). Then reattempt to mount. Keep doing this, and she'll eventually stand nice. IF she doesn't respond, become more aggressive with the backing or circling.
If she attempts to move off before you are firmly mounted, do the same thing as you did on the ground. If she's a real booger, and moves off when you have one foot in the stirrup and are swinging over, grab the rein and make her do TIGHT circles around. So mounting from the right, grab the right rein and make her circle around you while you swing your leg over. Then make her stand still until you ask her to move off.
It's a process, but eventually they learn its easier to let you get on then to do infinite circles/backing.