Hi 3image, unfortunately the Shell command in geometry library that MoI currently uses is very weak when it comes to shelling, it can't handle anything that would produce a different topology like if 2 vertices need to combine together or branch out to more.

You can usually get an idea of what in particular is going wrong by using the Offset command. To do this, select your object (as a whole object, not as a face selection), then run Construct > Offset > Offset (instead of Shell), put in Distance = 1 and then check the "Flip" checkbox, then right-click or push Done. Then delete or hide the original object and look at the offset result and you can then see the spots that look weird where there were problems.

In your particular case here it's also a problem that there seem to be some tiny slivers on the bottom face - if you hide or delete all the other things than bottom face you can see these little sharp points coming out of it:



As a side note, the anti-aliased edge display can help to identify areas like that because multiple edges drawn on top of each other will be darkened a bit as a side effect, it's subtle though.

- Michael