Caroleg

I need help! Audi A3 61 plate, starts fine, when I get back in, it won't start. Dash lights up, ticks over but won't catch. Have had diagnostics and detailed diagnostics done on it, only thing that comes up is key. Got key checked and ignition sensor, both fine. Left at garage for 3 days, they couldn't find any fault Its still happening - anybody got any ideas please??