Ah... you noticed that too. Same thing happen to me last year (2015). I won the away leg 2-1, but the game appeared to setting things up in the home game. It went to extra time and I lost on penalties. During that match, that is before going to penalty shoot-outs, my best player, a player who never missed a penalty, missed one. Everything was happening but the decisive goal to get my team qualified simply 'did not want to come'. And you know what, at that time, I was using 4-5-1(V); don't come and tell me it easy to score 2 goals past a superior team playing a defensive 4-5-1(V) in his stadium.
In some cases, the opposing team might even get one or a few red cards, but still you won't get to win this match or get a good enough result to go to the next stage. It is as if the game is trying to fool you.
There is something else I have observed. In some cases, where you have won the first leg by large margins or whatappeared to be a great result, in the second leg it is as if something has changed significantly; the other team start scoring for fun against you despite being inferior, playing away and kept the same formations and tactics as in the previous leg, while your players does not seem to want to play or they are playing but all efforts go in vain.