Cisco PVST to MST

[Jollywg]

I have 3 Cisco switches (catalyst 3600 series I think) that I am wanting to move from PVST to MST. Two of the three switches I can type "spanning-tree mode mst" and it takes it. This last one any time I type that it locks the switch up and I loose my connection, leaving me no choice but to restart it (which of course resets it back to PVST). There are only 2 vlans
1 -disabled and 2 everything is on this vlan.

Let me know if you need more information

Thanks for any help!

[sicarie]

Do the IOS versions match between the 3 systems?

I just moved and my catalyst is on a truck in Chicago, but I can try...