What are the causes of Adsense Ads not appearing

JS enabled in browser, yes, but Adsense doesn't work on several PCs

[Macro]

I first had it on one PC. It never showed Adsense Ads (IE 5.5). Then a PC that was showing the ads suddenly stopped (IE 6) and just won't show any adsense ads on any sites that have the code.

Now it's happened to a third PC at a completely different location (again IE 6.0 SP1). And I can't figure it out. Tried removing all the toolbars, twiddling with settings in Tools > Internet options etc.

Does anyone know of any reasons why just a blank space appears where the ads should?

Thanks

[Macro]

Thought I had the solution here: [webmasterworld.com...]

But no, it's not JS being disabled.

[Macro]

Been working on this.

On one PC clearing history in IE seemed to do the trick.

On another PC (win98se, IE 6) that didn't work. Taking IE back to an earlier version and then upgrading it back to IE6 SP1 seems to have solved the problem.

I'll try the third PC when I get there tomorrow.