Logic Pro 8 Vocals dropping out after bounce

I've used Logic express 8 now for about 8 months. I record and mix 1 minute spots for radio broadcast. Recently, I've run into a problem where the vocal I mix on site (not in the studio) drops out. I record with an RE 20, an RE 50, API Pres directly to a Sony DCM 50@24/96. I process the voices, bounce to 24/44.1 and mix with a canned music CD @ 16/44.1. When the voices are bounced to 24/44.1, they sound perfect. They're all strong, and even, my ears and the meters don't lie. But the second bounce (down to 16/44.1) after adding canned background music results in the voice recorded on site dropping out, I mean really dropping out -15 to -20 db. I have no clue as to why, and wondered if this is a problem with Logic express 8. Previously I recorded these vocals with a Sennheiser MKH 60 at 16/44.1 with no problems at mix down whatsoever. I have a suspicion that the bounce algorithm is causing this. If you have any suggestions please let me know. I want to record at 24/96 as it is noticeably higher definition on the transcients even after a bounce down to 16/44.1.
I also mix for radio broadcast, and I usually find I have to pull the level of any music CD down at least 10-15db against voices, even those recorded in studio. Your post makes it sound as though you are simply bouncing the files together, please forgive me if I've misunderstood.
How are you mixing? Ears (and even meters) will lie if the recorded voices are played in isolation without reference to any other sound; the essence of mixing is to adjust the levels of two or more signals in relation to one another, so if the music is overpowering your voices then you have not mixed, you've just combined the two signals. If you're not having a problem with your studio-recorded voices but your remote-recorded voices are low, then you'll have to compress and raise their level (and possibly pull the music back) until the mix sounds right. HTH
Thanks for your reply, I mix the voices with the background music, not only do I compress and raise the levels (I write the volumes with automation, in this case word by word) I also add some 'verb, slight delay and expansion. Mixing is not new to me, I owned a small studio in Orlando for 5 years, but that was Pro tools mixed on SCSI, using all out board compressors, reverbs etc. Oh, the good ole days! The strange thing is the Commercials sound fine on both my Mac Book Pro and IMac. I've never experienced anything quite like this, that's why I'm stumped.
Ah, sorry to doubt your experience. One thought occurs to me though: Are your field recordings imported into Logic as stereo files? I had a video editor here struggling with much the same symptoms, and it turned out his sync recordings were coming in as stereo files with the phase out 180degrees between the two channels. Everything sounded fine when played back on stereo speakers, but when summed to mono, the sync completely disappeared. It could be your on-site recordings are being recorded as stereo files on a unit that mistakenly has the left and right channels out of phase. Just a guess...
Thanks - Sonny,
That's the ticket, although the problem was a little more complicated. I did not sum the vocals into mono, so the phase cancellation did not occur there. But the room I record in on site is only 6ft X 10 ft and is totally untreated. I had recently switched mics from a cardidoid pickup Sennheiser 418 shotgun mic to an omni pickup EV 50. So the reflections off the walls were picked up by the omni mic and threw it just enough out of phase that when the music was mixed, they dropped out. When I inverted the phase, they popped right in your face again! THanks!