You need to "bed" or condition the break pads for proper grip. Look for the procedure in your documentation for the breaks.
IIRC, you need to apply moderate break pressure while running the engine at a high power setting while taxiing slowly for about 500 feet. This will heat up the pads and disk. Then allow everything to cool down.
Next, perform a full power run-up and see if the breaks hold. If not, perform the bedding procedure again, let cool, and then the full,power run up. Should be good to go at this point.
It is best to do this procedure with the wheel pants off.