Why didn't anyone go check up on AIVIS after the 1st Pass?
Sariel's comments about posts are added in green.

Susi, 9/7/98

Why didn't anybody return to Landing to check the AIVAS during First Fall or after it? I think it highly unlikely that people would disclaim such a useful facility. Somehow the explanations in the books were not really satisfying.

Jeremy Wong, 9/7/98

I think that after Mt Garben erupted, they could not locate the AIVAS building without a reference.

Cela, 9/7/98

Things were pretty manic during the evacuation - people were to busy trying to survive.  The conditions at Landing were dangerous - poisonous gases and such like - maybe keeping Aivas operational was just too difficult. I think Sean or Mihall went back to check for Ezra Keroon when he was dying in the plague and found the building buried under ash - they probably decided it was too much trouble to dig it out.

Sariel, 9/7/98

During 1st fall people were too busy with thread to have the time for it.  Afterwards, either
A)they'd forgotten already; 
B)the volcano was quite probably still active enough to keep them away; 
C)they couldn't find it under the ash;
D)the location looked like it would have been scorched beyond repair by lava. 
Any number of reasonable explanations can be postulated...