help with SRAM hydraulic disk brakes!!

So I’ve just bought a Giant TCX advanced SX and I noticed the front disk brakes didn’t work very well ( after breaking in screeching and lack of power, not being able to lift the back wheel in full force braking) I took it back to the shop where they replaced them with new (different) ones. These also don’t seem to work very well and are also screeching. I am very new to disk brakes as I’ve came from a race bike with rim brakes although I have ridden disks before and been very impressed with the power. What could be causing this? (bearing in mind they are hydraulic disk brakes)