The only time I've seen this come up without actually calling it myself is when a block is inserted that has a nested block in it. In other words, the block needs to be exploded in order to edit the attributes. It is like inserting a circuit, if you use the normal insert for inserting the circuit, the circuit comes in and needs exploding to function properly. The insert circuit command does this automatically and more. I'll bet that the block your having issues with is similar. Try exploding it and go from there. You may have to go into the block drawing itself and see if it is a block or the components that make up the block. If it is a block, then explode it so that it will go in a block instead of a nested block. Good luck and I hope this sense or helps a little.