Are these things actually 'hidden', or do they have some 'off' [aka 'hidden'] layer[s] assigned to them ?
To see if they are hidden use View > Hidden Geometry > ON...
Otherwise, check the layer settings...
Check which layers the vanishing objects might have assigned...
Obviously, SketchUp should not be deciding to make somethings 'hidden' [in either sense] in an unexpected way...
I'm sure we can find the culprit...