I can easily adjust my rear view mirror from left to right (i.e. in the horizontal plane) but it seems less willing to be adjusted vertically - there is no movement at all. Is this the same for everyone else or am I not pushing hard enough?

I can adjust mine vertically - but it takes far more effort than doing a left to right adjustment. As you have found, it is quite stiff and I found it best to use both hands - but it does adjust.

Same here. The upside is it does stay where you want to it to!