Lately I have seen N/A against some of the application defined in our initial TPX Main menu, even though the applications are active. After few minutes, the N/A goes away.
TPX uses the VTAM INQUIRE APPLSTAT macro for the current status (ACT / N/A ...etc.) TPX populates this field based on the reply from VTAM.
If VTAM does not respond quickly enough then TPX considers this to be a negative response, and marks the application not available on the Main menu.
The SMRT parameter, Inquire Checking Interval, may need to be increased, if you notice too many N/A displays that represent incorrect status for applications that are known to be active.