Bicycle: BMX Bottom bracket clicking / slipping while peddling hard?

What could cause my bottom bracket to click while peddling and if I pedal hard it slips. The chain is tight enough and straight and it's not the pedals clicking. I'm 90% sure it's the bottom bracket which is slipping, not the chain. The bike is brand new but I can't return it because I already change the handlebars, seat and grips