Hi GA,
Had exactly the same problem on my 1st couple of runs and was ready to give it up as a bad job. I think the reason is you have to keep the crosshairs perfectly stationary on the target while pressing the "y" (Default) key. Make sure you are zoomed in to a maximum (Using "[" & "]") that appears to have been my problem at least. ( I think if you are not zoomed in fully any movement is exagerated (sp)? and that stops the calibration completing). I use Kwassa's tutorial and it works fine now. (Although I do find I get more accurate results if I open the doors before anything else).
Hope this helps
TTFN
snafu