Speaker placement behind the screen

If your theater floor slopes upward to the rear, a speaker 3' off the floor will be well below the ears of many of the viewers, which could cause a decrease in volume unless the speaker had an unusually wide vertical dispersion. 

Please give us more info on the room: screen size, seating configuration, source material (blu-ray vs DVD), sound system especially amplification, decoding of sound tracks (Dolby, HD, DTS, etc), height of the seating (front to back), and the exact screen material. Lots of variables. I just had a custom screen made for my room. Expertly done. Acoustically transparent. Gain 1.1  All these things are important. Best of luck. 

additional center channel amp for dialogues may fix problem.

It is hard to tell without speaking to the OP, as some one who has worked in motion picture sound I have some thoughts:

Take care of the basics

• Horns are positioned facing the middle of the audience, not the back wall and ceiling.
  

• Measure your speakers at multiple locations and apply corrective EQ as needed.
  
• Control resonances to modern standards.
  
• Minimize or eliminate noise such as A/C units, outside sounds.
  
• Ensure all 3 speakers are operating optimally. Horns and woofers are both producing correct output.
  
• Ensure your overall speaker levels are properly calibrated.
  

Now the next part will seem counter intuitive to audiophiles, and I'm sure there will be uninformed arguments:

• Some movies such as LoTR are produced with excessive dynamic range. To hear the softest voices requires too high a volume, which you then suffer through far too loud effects.
  

The solution is to limit the dynamic range. I'm not sure what pro sound processors now use, but investigate and enable any compression that is available to you. If you can't find it via the Dolby/Datasat settings, an audio compressor on the center channel may be what is needed. Available through many recording gear outlets.

The compressor will work to raise the level of the softest sounds, while leaving audible levels alone.

Thanks to everyone who responded.  Getting closer to a solution, and will probably contact a sound tech at the local commercial cinema.