Some magnetic materials require what is known a 'Keeper' to complete the magnetic field, if the armature is removed, they can lose magnetism, probably what happened to you.
Many servo motors use rare earth magnetic materials now that some manuf. claim do not need a keeper when servicing.
It is seemingly more common with steppers to lose the magnetic strength if no metal 'keeper' is used.
A common example is a simple horse shoe magnet, they generally have a metal bar across the poles to help retain their strength when stored.
For $25.00 you should have bought a couple while you were at it.