what aux port are you connecting the camera to?
ch9 -> aux1
these are now called SERVOx in the parameters (instead of chx) (plane v3.7), ensure you select the correct channel and use SERVO9_Function = 10 (for example).
then go to initial setup-optional harware-gimbal and check if trigger tyep is enabled and with the correct output channel.
also, if you are triggering using SERVO remember to press the SAFETY SWITCH.