Ensure you have heading, that the speed isn't too fast or slow. If you have tide graph in the NavData boxes; remove it. The tide graph is reported to be causing some machine slowdowns with v3.70.1 but will be addressed in the next software version.
You need to be inshore enough to be near a tide station. Hit HOME - TIDE to see the nearest tide station on the fly.
Heading and GPS position are a requirement to record PBG. Normally the heading source like your autopilot should be turned ON. Heading can be checked in a data box to see if you have numbers.
PBG requires good signal and speed of the boat and other factors can keep it from recording. PBG is recorded to the memory card that has your mapping at the back of the machine. It would be recommended to ensure you have enough free space left on the memory for storing data.