Triggers can be used by both Navigational and SQL, However what is not specified explicitly is that triggers only work if the table is opened via the Data Dictionary since triggers can only be defined via the Data Dictionary. Unfortunately if a table is opened as on a Free Table connection, the connection (and server) have no idea about the trigger and won't execute it..
Clipper is not able to access tables via a Data Dictionary. It can only Authenticate (for AIS Purposes), but all table access is via Free Table access.
If by any chance your app is still clipper code but compiled with xBase or xHarbour you should be able to make a Data Dictionary connection and open the table via the connection which would allow the trigger to fire.