What Mario said. This happens most often with bass because some bass parts were written or recorded into a midi file with a 5 string bass in mind. Those can go to the B below the low E. If your synth doesn't have a 5 string bass patch with samples that go that low, you hear nothing. When you look at the actual notes see where they fit on the bass clef. If they're not those low notes then it could be the polyphony. You can check for overlapping notes by looking in the piano roll window.

Bob


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