First of all, you completely skipped the support template — without basic information this can’t be properly addressed.
Your post is missing the Nextcloud version, installation type (Docker, Snap, classic), PHP version, database used, list of active apps, and any relevant logs — without this, it’s pure guesswork what might be causing the issue.
At least check the following points:
- whether the users are in the same groups
- whether “restrict sharing to group members only” is enabled in the sharing settings
- whether the new user has logged in at least once (otherwise they might not appear in the sharing search)
- whether you have apps like Circles, Group folders, ACL installed which could affect sharing
- whether indexing/cache might be causing a delay (some changes only take effect after a while)
Without the missing basic information, it’s hard to give any proper advice.