My first guess would be exposure time. If you have another board ready, use it for testing different exposure times. For example you can expose only a part of the board for one minute, covering most of it with some cardboard. Then you move the cardboard slightly, exposing more of the board to the sun, wait one minute more, move again, expose more, etc... and at the end expose all of the board for 3 minutes.
Then at the end you'll have a part of the board that has been exposed for 3 minutes, a part that has been 4 minutes, etc...
Then put the board in the bath and see how the different parts reacts to find the best exposure time. I agree it shouldn't be more than 5 minutes to develop it in the bath.