When prism is installed and set to use OPOS, the original SO object in the registry (HKEY_LOCAL_MACHINE/SOFTWARE/OLEforRetail/ServiceOPOS/POSPrinter/Name of Printer (append WOW6432Node after SOFTWARE for 64 bit installs) is backed up to a new key named, "aDefault". The prism SO value than takes of the key name "Default" and acts as a passthrough from the POS application to prism and then to the printer itself.
If you reinstall the Epson ADK that comes with OPOS installs (CheckHealth etc), then this value is reset and prism no longer works. To fix, restart the prism service after installing the Epson ADK.