This has nothing to do with the interleave setting, which would not affect whether or not the envelope was being applied. If the pan slider isn't moving, then the pan envelope isn't being applied.
This may sound like a dumb question, but are you sure the envelope is actually assigned to the pan function?
Another question we've neglected to ask: is the pan envelope on a MIDI track or an audio track? It's possible, for example, to direct an automation envelope to the synthesizer's pan control rather than SONAR's pan control.