Power output from your WaterLily is directly related to the flow rate of the water you are using it in. If you are not getting the output you require, you may need to adjust the placement of the turbine in the water or find a water source with a higher flow rate.
For some tips on how to set up your WaterLily for better output please see the links below:
8 Tips to Effectively Charge Devices in Water
WaterLily Setup in River