A long time customer of ours (good guy, has poor taste in baseball teams) emailed me a drawing in which he could not explode the bulk of the building. Some research produced a solution – the block was specified to not allow exploding when it was created. This, fortunately, is easily correctable.
A LISTing of the block gives the hint as to the cause:
Notice it says Allow exploding: No
This can be changed by typing in B or BLOCK to bring up the Block Definition dialog box. Then, select the block you want to explode – in this case it is NORTH COMPLEX, as given by the LIST command, above.
Notice that the Allow exploding check box is deselected. Select that box to add the check mark and pick OK.
You will be asked to confirm the redefinition of the block. Select Redefine block.
You will now be able to EXPLODE the block at will
The H-Factor 
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