Miing Tsai pressure Cooker

Just got my pressure cooker recently… like it, but i’ve made two things in it so far, rump roast following the 

instructions exactly- came out dry and rubbery.. so disappointed since I thought it would be fork tender.

Anyone know what may have gone wrong? The pressure cooker was working properly, and there was plenty

of liquid at the end of the 75 minute cooking time.