Hello everyone thanks for posting about the core and weekly challenges in Ubisoft Connect.
Which challenges, in particular, are you having issues with? The exact name of the challenge would be great to know.
All Ubisoft challenges and any game stats can sometimes take up to 24 hours to synchronize in the overlay/launcher and synchronize your progression and kills etc, though community challenges have been known to sometimes take longer to synchronize your efforts and involvement in the challenge too.
-Ubisoft Support