A simple remedy for this would be to increase the audio level of your center channel. This can usually be changed in the settings menu of your home theater receiver unit so just look for a "speaker level" selection and tweak sound levels accordingly. Some HTIB (home-theater-in-a-box) systems also have a function to enhance dialogue levels--Panasonic and Sony, for instance--so enabling this mode could be an easier fix.

Another way to improve the clarity of the dialogue in a motion picture soundtrack is to point your speaker toward your listening position (also called toe-in). Some center channel speakers tend to have feet that angle them too high so you may want to place some cardboard or Styrofoam at the rear to ensure they are projecting sound straight at your main listening position.