Hey mmechatronica , it could be that the mirror modifier is going around the object's origin point (the default). Try mirroring things around the door frame object, which should have its origin right in the center.
Thanks Jonathan, yes that is "one" of the causes as well as the mirror modifier been applied to some of the objects causing them to move along in the same direction and another problem is that some of the parented objects are not moving at the same rate so when they move there drifting apart so Ctrl + J seemed to fix this drifting, although not sure whether this is a good approach?
I'd avoid joining objects together just yet if you can, since that step comes later. Try going back to the first frame when everything is as it should be, and unparent everything, delete the mesh that was created by applying the modifiers, and remove the modifiers. That way you can do the parenting and mirroring process again with a fresh start, since it sounds like something got mixed up along the way (parenting to the wrong objects or applying the modifiers too early). Hope that helps!