I haven't seen the Flex firmware code, but the product description says:
It's smart enough to automatically adjust the power draw from the USB port based on how many LEDs are controlled
USB can only deliver 500mA, so for 32 LED ~16mA. The specs on the LED modules say they can draw up to 60mA, so the Flex might be limiting power to the device and lowering overall brightness.
Are you testing with the same USB port and same length cable? Different hubs and ports could have different power ratings.