Hmm, this is very odd. I've never heard of a problem like this. Do the larger scale maps just come up with 'downloading' tiles? If so, do you get a message that TF is having trouble contacting Terraserver? Or, what happens when you try to view them?
Combo 1m works, but Aerial 1m doesn't??
Also, check options->preferences->tilesets, and make sure all the tilesets you want are enabled (checked).