Your beer may be good tasting to you. If your Diacetyl threshold is high, a judge with a low threshold will ding you for it. Same for DMS.
One technique I picked up from the guys in the club is to bottle more than you need for a comp. When you get the scoresheets back that indicate a problem, open one of those bottles and go over the flaws listed, and see if you can taste those flaws. A club is also a great way to solicit advice from folks who are BJCP, and see what they can tell you. If one says Diacetyl, and you don't taste it, consider that your palate may have a hole in it for Diacetyl - in fact there are a fair number of people that are taste blind to Diacetyl.
The friends in my garage are drinking free beer and I do not value thier opinions highly as they don't brew or judge. They always say it is great beer. There are some guys in the club that I seek out opinions about my beer, and that is valuable feedback. Scoresheets I will look through and often agree with and sometmes not at all.