I think that this is more of a question for the 3 Axis Machine Forum since it deals with Control Nesting. This particular forum is for custom tooling profiles used within eCabinet Systems.
In response to your question though - check your sheet material size. It could be that your back component cannot be nested within the defined sheet size. Remember that you have to consider the buffer or collar around the edge of the sheet. Another thing, if applicable, is grain and the grain direction on the part itself.