lil help with Blender please

I have been trying to export a couple files to stl and both have a similar issue afterwards. they are missing a couple mirrored halfs of the model. seems like maybe  designer copied/flipped like one side of wheels etc. they are complete in blender yet 1 side dont get exported. I am very bad at learning how to do things in blender so mostly only use it to convert blend to stl. when nothing else works.
any ideas what may cause this and how to fix it?

its like in other programs when you forget to lock/group parts together. I have seen other file types slip parts into wrong plane like merging or deforming parts of models. just been putting reluctant files in a folder to deal with later if I ever find how. lol