There are a plethora of possibilities that could cause this. Here are a few common things you could check:
1) Ensure that those two users have the same permissions to the list as everyone else. This involves exploring how all the other users are granted access (whether it is via a SharePoint Audience, a SharePoint Group, an AD group, etc.).
2) Try manually adding the two users to the permissions on the library.
3) If there isn't any data that would be lost in doing this, you could try removing their user profiles from both the User Profile Service Application as well as the Site and Site Collection. Then run a profile sync to have their profile re-added.
4) Try this on a different Site with a different Library. Give the same permissions you gave to the main form and see if the same issue occurs.
5) Check the Event Viewer and ULS logs for any errors that occur at the time the users try to access the form.
6) Check any data sources that the form relies on to ensure the users have permissions to those.