Agree with the above comments, but if you are after a laugh...
Could it be that it requires you to make a placement grid setting that is not small, on a 1th, or 0.05mm grid etc there will be thousands of points it has to consider so it could be that it is just taking a long time to think.
CADSTAR does this, appears to be doing nothing when autoplacing on a small grid - change the grid to a larger one and it does it in seconds.
Also, have you placed and parts such as connectors etc. ?? these allow the autoplacer to check and see what else is connected to them then place them.
I suspect that Altium is sat there scratching it's head thinking which component to place first.