One possible and common reason is syntax itself - all distributions of latex do not use same syntax and what works in mimetex does not always work in TexLive/teTeX etc and vice versa.
And for example wiki is a special case where you may need to disable CamelCase to render all images and for example square bracket tags always fail in wiki if you add latex tags.
You can also save problematic images to moodledata as normal images (no matter what math editor you take them from and add those eguations with image plugin - or use external latex renderer...