Be reassured, that as a UK native speaker, I have to listen extra carefully - and even rewind the DVD - to understand some of the dialogue at times in Hollywood movies. They are too busy being 'tough, rough, and this is me, how raw and natural I am' that the words are mumbled or swallowed.
Then again, I was in Thailand with free in-house Hollywood movies in one of the hotels. Despite the impeccable quality of the picture and sound, the subtitles (I had no choice), made me suspect they were pirated. If some of your DVDs are the same:
The most recent of the Harry Potter movies they showed (just opened in the cinemas) was more entertaining comparing the spoken English dialogue with the English subtitles. One I remember: something about 'a torch seen in the wood', out of the blue, whatever the conversation had been, became something about 'a touch-screen TV"...