Jenny,
If your Belkin KVM was one of the ones that came with a serial management cable then there are some firmware updates on Belkin's site that address this problem. If your switch has the necessary port then you will need to locate the special lead that came in the box and update the firmware from a Windows PC.
They are actually offered as a fix for a particular range of IBM servers and a seemingly unrelated issue, but I have found them to resolve the missing mouse problem at a clients before.
In my tests switching to USB only made the problem worse.