No resolution on 40X?

[swiftguy]

Every teacher has had a student who has asked for help because he/she could not get a clear image when switching to 40X. Possible reasons for this happening are:
-contaminated lens, usually caused by previously working with wet-mounts. Lens needs to be cleaned.
-prepared slide is upside-down. It will work on 4X & 10X, but the cover glass is too thick for viewing on 40X.
-objective is loose, make sure that it is screwed in tight.
-spring-loaded lens is stuck inside objective. This should only occur on old or very dirty scopes.

Other causes, or helps?