Random parts of the model vanish

I'm having odd issues when I upload my sketchup files to shapeways.

It goes something like this- I will upload the file to check it and Shapeways will suggest that some part might be too thin so I push pull it to be thick enough.

Then when I upload the new file parts of the model have vanished even though I have not modified them or moved those object points in any way- it is as if adjusting a point that is no where near these tires has suddenly prevented Shapeways from seeing those tires.

This seems to happen often and I cant tell if it is Shapeways or Sketchup (2016 Pro) but it is maddening- fixing the tires doesn't always help as some times it likes the new tire- toher times fixing one tire will have some other tire go wonky- there are no random object points hidden within them and they are all watertight so it isn't a simple issue like a random line within the insides...

Shapeways Contact Us help says they have no clue and recommended I come here :/ or else hire a designer to fix it (as if )

Sketch up has not gotten back to me but judging from their facebook page bugs and glitches are common enough and I am wondering if perhaps the solution isn't to move to Blender or some other program?

Try checking the model with the SolidInspector plugin for sketchup, or check the exported file with either meshlab or netfabb studio basic.

thank you- well it did identify errors- but I'm uncertain as to why they are errors as I created this following the instructions in the demo vids :/

Worse Solid Inspector says it cant fix them ?

How are these errors being formed?

image attached

well I found two parts where it wasn't quite water tight so that answers my own question- it WAS water tight at one point but it seems SketchUp dropped those surfaces at some point- once filled then Solid Inspector WAS able to fix the errors remaining

I'm still a but nonplussed on how the remaining errors formed? Internal face errors are ones inside the model? Is this something common to SketchUp or is it typical with all 3d programs?

Edit- well seeing how Solid Inspector fixes the models Ive figured out that it is indeed those internal walls- a solid wouldn't have any obviously just one big vast space that is 'filled'?

WOW- let me just say that they really should make Solid Inspector part and parcel to Sketchup- without it one would go mad looking for errors- with it you can not only look for any errors it becomes obvious how they do indeed sometimes progress as a natural result of building your model.

In all honesty I wasn't going to recommend Sketchup at first- certainly wasn't going to buy a version after my 30 day trail: though it is super easy the buggy factor of any larger objects would have made it insanely time consuming- with Solid Inspector it shows you WHERE the error is (highlighted in red) and its now become a matter of course for me to 'check my work' every few steps to make sure it hasn't accrued any errors.
To be honest if Google hasn't hired the fellow who made it they are mad as SI makes the program redeemable

MANY thanks for your help- TOTALLY made the difference to me