I tested this feature for the MSHTML team about 12 years ago, but not for complex scripts. Are the client's machines ones that have Complex Scripts support installed? They need to have that support installed in Windows (Uniscribe in particular, and supporting tables in NLS in general) for IE to deal with complex scripts.
It would be helpful if you described your specific problem.
I don't think Weft did much more than create a minimalist version of a font wrapped inside a special format. But it may have been missing some truetype tables that are essential for complex scripts. I doubt my old contacts on the complex scripts team at MS are still involved
in IE. Michael Kaplan might have some suggestions; you could try his blog. http://blogs.msdn.com/michkap . He has written a bit about WEFT from time to time.
It's also possible that support for WEFT has changed a bit since IE8, so make sure you clarify which browsers you've tested with, and what language support is installed in the client machines.
You may also want to specify DIR="RTL" in your HTML on any Arabic or Hebrew regions, or on the root HTML element, especially if you are using UTF-8 as your encoding.