A similar question was posted on AVSforum, so here is my reply:
Probably the panel has buckled a bit with age and the front to rear seal along the edge of the two stators has separated along that segment leaving no contact path for the signal. There is a copper band that the diaphragm is supposed to be in contact with, but if the seam between the front and rear stators is loose, no contact, thus no sound.
Take the rails of the SL3 and check the panels. Close the gap, then re-seat the rails so they properly press on the panel edges to ensure good contact. The rubber strips on the frame are likely compressed and failed with age, so consider replacing those to help ensure a tight fit. Plenty of tear-down and fixit advice over on the MartinLogan Owners Club site.
It is also probably time for new panels on units of that age, thankfully ML still stocks/makes them and will sell you a pair of brand new panels. Downside is they are not cheap. But will last another 20 years.