Thinking about this somemore while making a mess in the workshop - ours doesnt go particularly fast, nor dive fully on the hydroplanes only. I think this may be normal, as to get a dynamic dive would need the sub to move quite quickly.
Regarding the dynamic dive, the battery is only a 3a/hr battery, so wont last too long if you dive lots and run it at full speed. We tend to dive to just below the surface - any deeper and we lose sight the water is so mucky!
Make the checks I suggested, but it may just be behaving as expected, unless someone else has better experience.
One other check I recommend is to put it in the bath without the yellow plsastic outer shell, run the pump to fill the bag and make sure the overpressure switch operates to stop the pump.
When we got ours I tested it in my tank and the bag split, filling the watertight tube. I tracked this down to the pressure switch failing. If this had happened in open water the sub would have been lost.
Ian