Can you spin the rotor by hand when the motor is turned off?
I had the same problem with an older motor were the rotor could spin around with the motor turned off but the moment I hit the on switch the rotor slammed into the stator and got jammed against it. Maybe the stator mounting hole for the rotor is less wide than later motors or maybe the rotor is slightly bigger than later models, I don't know for sure, I do know that there are some differences within the motors series with some of the rotors being somewhat bigger than others.
What you can do is to centre the coils as described here; http://www.lencoheaven.net/forum/index.php?topic=2088.0
, that worked for me but I had to be very precise when tightening the screws, the width between the stator and rotor since the gap between the stator and rotor with this motor is less than a twice folded paper sheet at any point.
When you pull out the rotor, check the stator for any debris in the rotor's mounting hole and clean it out.
Since you already have the motor half dissassembled at this point you propably want to lubricate it as well, it's just maintanance
see here for the details; http://www.lencoheaven.net/forum/index.php?topic=2087.0
But you can try to give it a little whack at first, when switched off, maybe the bearings will realign properly and your rotor wil be free to turn.