What kind of FD mount is your lens? The older one is the breech lock with a silver locking
ring. The newer one is a bayonet style.
Here is a comparison: http://canonfd.farah.cl/CanonFD_faql.html#mounts
Both lens mounts are compatible. The camera mount is the same.
The new bayonet style lenses can have some play on the brevis mount. They lock in place,
but you can twist them 1-2mm.
All my new style lenses are slightly loose on the brevis - but mount without problems on my canon cameras.
This was reported here on the forum previously, but there was no solution I'm aware of.