Thanks for your comments. After reading them and thinking more, this might be what happened:
Since I did not move the gun once I got a hit, all 5 rounds hit the same place. That place seemed to be the side armor below the turret, but above the track. So maybe this location, even though hit repeatedly, is just programmed to be invulnerable to 75mm shell fire. However, one would think that HE shells exploding next to the turret would have, at the very least, knocked it askew putting it out of action. I think LtHans's point about the weakness of the Ostwind's turret is important. I didn't expect to destroy the Ostwind, but I did think I could damage it. But maybe I just put all 5 shells into the wrong part. Bloom25's comment would seem to support this idea.
/S/ MRPLUTO, Captain, VMF-323, "Death Rattlers"