Oddly enough, I am experiencing the same exact issue (as Desmund above) right now, in Logic 10.6.3. I went into my mixer, and made sure my "Preview" Channel didn't have the solo button slashed, but that didn't help.
Basically, when I play my session & want to preview any loops, none of the loops play. If I stop my session & play the loops by themselves, they work, and I see/hear the audio coming through the Preview channel. But this doesn't occur while my session is playing. Is there something I am missing or not doing? My loops were working perfectly fine a few days ago (when I previewed 100's), so I am not sure what I've done to mess this up.
What I am working with: Apple iMac (10.15.7), Logic 10.6.3, and a Focusrite Saffire Pro 26.
Note: I've opened other song sessions, and the loops work fine. So obviously I've obliviously changed some setting in this particular session that has muted the loops.
UPDATE: I opened a bunch of other sessions, all of which were working fine. I noticed that ALL the "prelisten/preview" channels had their solo buttons with the red slash, and all working fine. After that, I opened my session in question, and all of a sudden the loops were playing along with the session again. Go figure! Any professional opinions about this?