In most of the cases, the reason behind receiving fewer fields than those expected in your API responses has to do with the endpoint URL used.
If the msgtype parameter is not defined and included in the URL, the response is set to simple by default.
For example, a PS02 - Vessel Positions API call using the following URL
would only return the fields contained in the simple response (MMSI, IMO, SHIP_ID, LAT, LON, SPEED, HEADING, COURSE, STATUS, TIMESTAMP, DSRC, UTC_SECONDS). The fields of the extended response would have been omitted (SHIPNAME, SHIPTYPE, CALLSIGN, FLAG, LENGTH, WIDTH, GRT, DWT, YEAR_BUILT, TYPE_NAME and AIS_TYPE_SUMMARY).
So, if you get fewer fields, make sure that the endpoint URL you use for API calling includes the msgtype parameter.
The actions mentioned are applicable to the following API Services:
PS02 - Vessel Positions
PS03 - Vessel Positions of a MarineTraffic Fleet
PS04 - Vessel Positions within a Port
PS05 - Vessel Positions in a Predefined Area
PS06 - Vessel Positions in a Custom Area
PS07 - Single Vessel Positions
EV01 - Port Calls
VI01 - Voyage Forecasts
VI02 - Expected Arrivals