Winforms Tooltips only work on certain machines?

Using Winforms, C# and VS 2010 I've made an odd discovery that tooltips only seems to work on certain machines, although it seems to be independent of OS.

They work on one XP machine but not 2 other开发者_如何学JAVAs, but work on a Windows 7 machine. Anybody ever seen anything like this?

I've developed across several OS's with several languages (.NET and otherwise) and I've never come across this. Must be a config issue.