When your center speaker is behind the screen like yours is ... you need a special porous screen made to allow the dialog to come through it
THX encoded DVDs actually boost the dialog 3db for this purpose ... it's called an "X" curve if I remember correctly
This 3db boost to the center channel was recorded into the THX certified DVDs so you could place the center speaker behind the special purpose porous screen and the screen still pulled down the sound pressure level (SPL) by 3dbs to balance with the front right and left speakers
The opposite side of the coin is if you play a THX certified DVD disk and place the speaker in front of the screen ...the dialog is to loud and must be dialed down in the processor which most HT enthusiastic do
Contact "Stewart Film Screens" and talk to someone in support .. they can explain it and help you out but you may have to replace the screen with the proper type for use with a speaker in back of it