To add a little to the previous post about what mode you are listening in - if you are using one of your receiver's built-in proprietary "music modes" (e.g. modes with names like "Rock Concert", "Jazz Club", etc.), many of these don't direct any signal to the center channel speaker. They just use the surrounds to add ambience.
As the previous post noted, DPL II does synthesize a center channel signal from 2-channel sources. But how much gets directed to the center channel will depend on the source material itself. Also, DPL II has a less pronounced center-channel mix for its music sub-mode, than for movies.
If your receiver has a "5 channel stereo" (or "7 channel stereo") mode, then this should give you more or less equivalent volumes out of all speakers, to at least verify that your levels are more or less correct. Using your receiver's built-in pink noise test tone will also tell you that.