At the risk of seeming impertinent, how old are you...?
Hearing deteriorates from the early 20s onwards, with the higher frequencies dropping faster than the lower end (for the audiophiles - the range from 4kHz upwards drops fastest). The effect of that is that as you get older it can be harder to pick out what people are saying against a general background noise, which is made worse if you've been exposed to high noise reasonably often during your life, even if only as a youngster going to loud clubs, etc. and then a life of quiet sobriety for 30 years afterwards.
In the English language, as in most languages, those 4 to 6kHz frequencies are where most of the meaning in speech is, with the lower end being all about volume and rhythm of the words rather than meaning. The effect of losing those higher frequencies is that speech can become a bit muffled - the classic example is sitting in a loud pub and you can hear perfectly clearly what's going on at the other side of the room or if a car has pulled up outside, but can't tell what the person sitting next to you is saying without really concentrating.
What you say about documentaries vs films makes sense. In documentaries there is usually very little background noise while people are talking, so the speech is nice and clear, while in films there is all kinds of music or sound effects going on and if people do have those high frequency losses then the speech gets buried in the whole noise and is harder to follow. Home cinema systems tend to be bass-heavy and can be harder to listen to than a normal stereo sound. The balance on modern TVs is also weighted towards the bottom end which again can make it harder to pick our speech above the general noise.
You don't have to be deaf to have this effect, it has nothing to do with poor hearing overall. As an example, not 30 minutes ago I had a lad in for a hearing test in his early 20s who has exactly the same problem and his hearing was excellent, just not quite as good at the higher frequencies than the lower.
As for solving it - no two people's hearing is the same and nobody over their mid-teens has perfect 'flat' hearing. That means the auto-settings on the system usually never suit a particular person but are a best-fit. Experiment with the sound settings to crank up the higher frequencies, just the treble if that's the only option, or everything from say 4kHz upwards if the system has fancier settings.
Probably just confuses everyone further but there you go!