Consider a few more things that could cause bitter or even sour may have been what you were getting.
How long did you warm the machine before pulling the shot and what type is it? Even if the machine said it was ready think again as most machines need 20-25 minutes at least to come fully up to temp and 40 minutes is not unheard of. On the time thing too not all shots over 30 seconds are bad either on my Izzo some shots of over 40 seconds have actually been really good ;)
Welcome too :)