This COULD be a permissions problem.
When you use a split database, the front-end gets copied to each user's machine. If the user performs the action to PULL a copy from the place designated for that purpose, then the user is the owner of the copy and should have Full Control (but not less than Modify) on the FE file. If the user didn't do a pull but instead someone did a push, FE ownership might not be right.
However, the BE file is shared. Each user must also have not less than Modify rights to the files and the folder designated for this purpose. A simple test is to have the user navigate to the folder holding the back end file. Right-click on the file. In the drop-down list, select Properties From the complex tabbed box, select Security. The user needs Modify permissions on the BE file AND on the folder that is holding that file. It would be clear from that screen immediately if the user had that permission.
Typically the person who would grant these permissions must have Full Control over the file permissions, which usually means an admin. In fact, the "Modify" general level of permissions is a composite setting comprised of about a dozen more specific permissions, but the Modify option contains all that you need to run Access.