Hmm.
Do you ever get an error message popping up? Can you confirm that topo 8m works? (Downloads new tiles and displays).
There are a couple things to try.
One being check your screen depth. Probably already 32bit, but if not, go to 32.
Second, in options->preferences, try changing the "Enable Texture Compression DXT1" option. Restart TF and see if you have any better luck with those tiles.