What do you mean exactly? The vague statement about issues won't help much.
Please let me know which version of UOFiddler you are using?
Do you use mul or uop file format?
Which client version files are you trying to modify/use?
Is it original client or some sort of customized format?
What is not working? Are there any errors or warnings?
There is over 20 different UOFiddler versions out there I can't really help you if I don't know what you are using
On polserver discord we have channel for UOFiddler the link is available in repository
GitHub - polserver/UOFiddler: UOFiddler based on Ultima SDK it's a tool to view and alter almost every UO 2D client file. Source code is released under the Beerware license.
It's worth knowing that after releaseing NL beta so client version 7.104.0 onwards. The clilocs, gumps and few other items were updated and UOFiddler will be unable to read those files and will crash.