[Koha] trouble placing hold on book

[Shelley & Kurt Kemperman]

I tried and failed to place a hold on a book today.  The book was  
already checked out and I wanted to place a hold so that when it was  
checked in, our volunteer would be alerted.  I went to the book  
record, clicked place hold, then entered the student's number.  I  
received the message "no copies are available to be placed on hold".   
Well of course not, if there was a copy available, I wouldn't need to  
place a hold - lol.  Why can't I place the hold?

Thanks, Shelley