When you press down you are manually holding the 'gas safety valve' open and in contact with an electro magnet. Once the flame is lit, the thermocouple warms and as it does so produces a tiny amount of electricity, the magnet requires a minimum of 15mv to energise and hold the safety valve open when you release the control rod.
So that's how it works, why it does not can only be one of a few small things
First is the pilot flame nice and blue and you shouldn't be able to see the tip of the flame looking through the viewing window? Yellow flames and small blue ones don't tend to heat the thermocouple as well and requires servicing to sort out.
Are you giving it long enough to heat up? a good strong thermocouple can reach 50mv so reaches 15mv and energise the magnet in 4 or 5 seconds, weak ones can take 20 seconds but any longer than this and the fires well ready for service.
Truma 3002 gas valves light anywhere but after '5' should be chosen to ensure the main burner will light so giving a clear indication the pilot has lit and by that fact holding the safety valve open.
Always, this fire or any other gas appliances should light without hassle or the need for any clever 'knack', if they don't it's an early warning something needs to be done to put it right, ignore these early warnings at your peril!