Too many activations

I bought several times the same ebook so I could tranfer it on 40 devices for all my students but the program dont reconize the difference and keep telling me I have  already transfer it too many times even though i haven't because it is a new and different file.

What can I do to solve this problem? Because I realy need to be the one making the transfer for the kids and I already spent my money on those ebooks.

Please help me!