Please specify where exactly you use it where it does not work: ODS, OAS and in OAS default processes or High/Low risk processes, etc... might make a difference.
Some other help in troubleshooting:
Also if your detection in that folder is a PUP that is not excluded by filename but detection name (just to be sure).
Also have a look at the following KB which explains wildcards quite well.
In your example I think that using **\foldername\** would do the trick
For instructions about wild card exclussions, please refer to KB#: KB54812.