I think I'd be inclined to run the SQL profiler t see what Sage is asking for when the Supplier list is called, then run the profiler again then you use the supplier lookup. That'll give you the SQL request & from that you can work out why the expected data isn't being returned, i.e. some missing related data. That's quite a lengthy task do though. I take it you have tried checking the database, rebuilding the indexes just incase it's something related to that?

Database?  Nobody told me anything about a database!

Unless you are using Sage 200 Standard or the Education variant I'd suggest talking to your business partner

Unless you are using Sage 200 Standard or the Education variant I'd suggest talking to your business partner