After every shot is interrupted your view through the scope and it´s impossible to see where your bullet actually goes.
When you click to shoot, keep holding the mouse button. As soon as you release the button your character cycles the action and of course interrupts the view, but you can choose when that happens by not immediately releasing the button when you fire.
For this reason you can´t change setup of distance on your scope and it makes difficult to hit target.
Page up and page down working just fine for me. Tried the zeroing in the arsenal at 100/500/1000m and it's spot on. Are you using any mods? ACE in particular?