The tools to help you resolve issues with non-solids... Solid Inspector will show issues but not fix them... Then in combination with an 'edit'... Layers - all switched ON [in case you have nested objects on OFF layers, or even worse! geometry on a layer that is not Layer0 that is off !!] View > Hidden Geometry > ON [shows invisible edges and 'hidden' objects] View in Xray mode [allows selection and deletion of internal edges] Section Planes [to allow you to access the inside of objects to remove faces, edges etc] Select + <delete> [to delete unwanted faces/edges] Erase [to remove unwanted edges by picking] Move [to select and relocate vertices to heal small holes etc] Line Tool [to over-drawn edges and [re]create missing faces] Remember that a solid group or component can only contain geometry. Every edge must have exactly two faces. So that means... No faceless edges. No edges around a hole [no matter how tiny]. No edges forming the unbounded side of a flap or shelf. No internal partition faces, which would then have some edges used by three or more faces. No otherwise seemingly solid boxes touching on a shared edge - which then has four faces. No exactly overlaid coplanar faces with shared edges - can happen [rarely] with some plugin operations.

Thanks for the help guys, really appreciated.