Hi Munira,
First of all, try forcing the submission forms to reload by restarting tomcat.
If that doesn't work then sounds like you might be experiencing something related to this issue:
https://github.com/DSpace/DSpace/issues/9402 where once you exceed 10 collections with a specific entity type, the entity-specific submission form would stop loading for new collections.
The issue has been fixed but you'll need to patch your instance with the linked pull request. If you've installed the code fix then you might have found a new bug.
If you're unable to incorporate the code from the pull request then you can workaround the issue by specifying the collection entity relationship in your item-submission.xml file. To do this edit item-submission.xml and add an entry like:
<name-map collection-handle="{add your collection handle here}" submission-name="OrgUnit"/> and restart tomcat.
You'll likely need to repeat this for each entity type affected. Let me know if this works for you.
Kind regards,
Peter