Computers disappear from application ins

While trying to install Inoculan to the client desktops, I had this problem. I went to Server Management Console> Computer Management> Install Software to push the client software to the desktops.
First you see a list of computers in the Computer Management properties window(all 20 computers show up in this list). Then you need to select the user to whom you are deploying the application. Then another screen ask you to select the computer from the list. Only 15 computers appear in this list. Ican’t deploy Inoculan(or any softeware) to the
computers that don’t appear in the list. Reinstalling the SBS client software didn’t help. Where does SBS get the info from regarding the computers? How do I fix it?