The “!” in your setup call is for “add every row to the log”. Normally the logfile is in the users temp folder %Temp% Have a look at the logfile may you find the reason.
Is your setup.exe a webkit or a regular install kit?
If it’s a regular install kit (with an .msi, .cab. etc all visable) then the -a command line will cause the installer to produce an admin image, not actually do the install.
If your setup.exe is a webkit (only setup.exe in the directory, no other files) then you might want to try removing the /qb+! and run the install in UI mode to determine if the install paths look correct. Also you’ll want to add the verbose logging command line option “/l*v %TEMP%\verbose.log”. If it does still not work you can use this log to examine what migt be going wrong and it’s the first thing IBM support is going to ask for.
Your command line would then look something like:
start /wait \dc2\install\lotusnotes851\setup.exe -s -a /s /v"/l*v %TEMP%\verbose.log"