You'll need to provide more info, when you paste does it say something like "some events could not paste because..."
What I do to copy and paste events is hold ctrl while clicking/highlighting the events, then right click and copy. Next click on an event where you want to paste in your event sheet, right click, paste. Be careful when selecting/clicking on your event as it should highlight the whole event and not just the conditions. Clicking and dragging actions/conditions can be tricky, but once you figure it out its pretty easy..hope this helps, good luck