Missing Porclaim Slides

We have used Proclaim for a year now and lately we are having problems with slides that have been inserted on one computer not showing up on another slide.  We normally have two people building the presentation.  We are now often experiencing a problem viewing certain slides (basicaly screen shots) not showing up on computers on which the data was not entered.  This missing slides are not consistent from computer to computer and not all of the slides of the same type are missing.  For instance, two weeks ago on a Saturday night I noticed that some of the song slides were missing (some were blank with a white background and many more had a yellow caution sign with a black exclamation point in the middle).  I email my collaborator who put those in and told him they were missing, but when he pulled them up, those slides were on his computer.  Yesterday my collaborator came to building and found many slides missing on the auditorium computer that he had inserted at home.  I pulled up the presentation on my office computer and there were a few slides missing, but not near as many as on the auditorium computer.  Usually there are only two of us invloved in building the presentation and we are not deleting slides.  So, why is this happening and how can it be prevented/stoppe?