Logic Pro: In 7.1 surround, my Logic project sends a lower level to speakers 1 and 2

Hi. I'm hoping someone can help me with this: In one particular 7.1 surround project of mine, Logic sends a much lower level to speakers 1 and 2 (regardless of where I place speakers 1 and 2 in the I/O Assignments). It does this for all sounds, audio and software. For example, if I send a mono signal equally to all speakers (from center position on the Surround Panner), the level is always much lower to speakers 1 and 2. FYI: When I open a different 7.1 project in Logic, the problem is rectified—equal levels to all speakers—and the two projects appear to be set identically (same Project Settings, I/O Assignments, etc.). But in the problematic project, I can see in my Main multimeter that speakers 1 and 2 are being sent a lower level; all other speakers are receiving the same stronger signal. (Incidentally, it does not appear to be my interface—an RME UCX II/Totalmix—since, as I say, I can see the discrepancy in Logic's Main multimeter, before it gets to the interface.) Can anyone think of a reason why a center-position mono signal would be weaker to just speakers 1 and 2? Thank you!