What is the color of the powder before the laser get active?
the convex bottom problem is a sign of warping and you have to change your temperature setting and to check that the IR sensor is working right.
Warping happens while part of the matierial fall under a spezic material temperatur and the material shinks at different point of the print. I hope you understand what try to say. My english is not so good.

What is your chamber temperature? (i take heat up 149,5 Printing 150) What is your surface temperatur? (166 / 170,5) What was your heat up time?