I have run into this issue many times. Unfortunately, I have found no definitive cause and thus no definitive way around it.
You could try to forceinstall a new agent over the current and see if that works (it did for me sometimes) or just, in this case, "touch" the 2 systems and perform the upgrade manually by copying the package from ePO.
Hope this helps.
Not too sure if it a similar issue but I have had systems not upgrade from VSE 8.7 to 8.8i. For me the best method is to first send a task to uninstall 8.7i and then send a task to install VSE 8.8i.
Sometimes you get an installation error that's not explained on McAfee Agent's log (not enough space, installing on a non-physical drive...). The best way to troubleshoot it would be installing manually VSE through terminal server (or on local console) and see the real error message