This will be because of the fact that you renamed and/or copied it. The original mission will have been accompanied by multiple script files. By renaming it straight as an MP mission you are losing those script files.
So copy your mission, make changes and save it as what ever. The open up your OFP diretory and copy the scriptfiles ect from the original mission to your new one. Only then, i think, can you save it to MP.
-supershooter