Keep in mind the following
The secondary locking must always be open.
Inspect the locking tab/tabs on the terminal for damage and check that they are not pressed inward.
The cable terminal must be inserted from the housing's cable side.
Check against the wiring diagram that you have the correct cavity/position number.
Turn the cable terminal the right way up. Check against the other terminals.
When the cable terminal is inserted in the cavity a "click" is often heard when the primary locking engages.
Check that the cable terminal is firmly in place by pulling lightly on the cable.