Thanks for reporting the problem. I think I see what you're talking about. Some tiles, apparently random, come in with "no maps available."
I am fairly sure this is a terraserver issue, and I'll wait to see if it gets resolved for the time being. One thing you can do is close TopoFusion and reopen it. This should give those tiles a "second chance" to load correctly.
I've definitely never seen this before coming from Terraserver. . .