This model has undergone many changes, been copied and saved as a new drawing several times, and at one stage having some loops attached. Those loops were deleted a couple of iterations ago along with other changes, but they appear when the model is exported as an stl. The model was saved by itself to a new file so there is nothing else in this file, and I have purged. It’s fine when exported in all other formats.
It is hard to tell what is going on only seeing the pictures.
The best would be if you upload your Sketchup model here, (or share via cloud storage if it is too big) , then someone can take a look on it.
… are hidden.
(Click on the picture if it is not animating)
That’s embarrassing! I made a point to “unhide all”. but didn’t check the out liner.
…does not unhide inside group or components. You have to open that group or component to unhide. Or use the outliner… 
There’s no option in the exporter relating to hidden objects, so not thought about. Something I’ve learned today. Cheers
Fairly sure I did that, but perhaps not. I very rarely “hide” things, but move them instead - because I forget they are there LOL.
