Zuludath

I played M05 three times and all ended in "Mission failed, didn't intercept in time" The first time, I didn't find it, second time, I locked him on radar and about 20 nm out, the third time, I intercepted it and just fired the warning shots as instructed but still it showed "didn't intercept in time, mission failed". Is this a bug, or I have to finish the whole intercpet and divert in a certain amout of time?