If you preselect elements in the floor plan and then switch to the 3D view
(using F4), AC10 and AC11 will remember those elements as a preselection so next time that you switch to 3D it will show only those elements. To verify when this happens either hit F4 (F5 in windows) while in the 3D window to force AC to clear the previous selections.
I am guessing that this feature might be your problem, since restarting AC will clear the selection and that will show the whole model in the 3D window.
Eduardo Rolón AIA NCARB
AC27 US/INT -> AC08Macbook Pro M1 Max 64GB ram, OS X 10.XX latest
another Moderator