Hi,
- Do you put the PCB in the tank and keep it still?
I get best results if I move the PCB through the fluid. Looks like it forms a small layer of corroded copper that stays over the board. When you move it through the fluid, this layer goes away allowing to a new layer to be corroded.
- What is your room temperature?
The ferric chloride works better if it is a little warm. Here where I live I got like 10°C of room temperature. What I do is use a small amout of corrodent in a small plastic bowl. So I put this small pot in a bigger one filled with boiling water. The water heatens up the corrodent and the proccess goes a lot faster.
Yesterday I made a PCB like yours in size, and it took only 10 minutes.
Hope this helps.