I had a feeling that I've seen this before on older versions when picking a certain criteria condition but I couldn't get this to go wrong on 7.5 when I just tried it. Perhaps it may also be down to whether your database collation is set to be Case Sensitive or not, mine is Case Insensitive.
Anyhow, I think a good place to start is to compare the criteria conditions between a query which is working the way you want it to and one which isn't. Within Query and Report Designer modify your query and then on the 3rd screen it should show you the criteria to compare.
Does this show anything which could be adjusted?
If you are using Oracle then I'm afraid this is expected behaviour unless you apply the wonderful case insensitive trigger obtainable from your local support team and take the advice they will offer you on index creation. With SQL Server as Karen points out if you are using a slightly unsupported character set or collation sequence then adjustments may be required before normal service is resumed.