Altium: Can't move vias
Generally when you are unable to drag components (and they're not locked) it's because clearance rules are holding them where they are. You can change the preferences in the DXP menu for moving components, but a less-permanent solution would be to press the "R" key while trying to drag the via. The "R" key switches the component placement mode between "Ignore Obstacles", "Push Obstacles", and "Avoid Obstacles". This should allow you to move the via regardless of the clearance rules provided you have the placement mode set to "Ignore Obstacles".

UPDATE

I just had the same problem today while routing a board for work. It is caused by clearance rules being broken by the traces attached to the via. What you must do is move the traces directly connected to the via first to make sure they are not breaking any clearance rules (or change the clearance rules, if possible), and then you should be able to move the via.