Thank you for reaching out, and sorry about the trouble.
Please let us know what happens when you try to combine the PDFs. Do you get any error message? If yes, please share the screenshot of the error message. If you are using the desktop application to combine files, share the Acrobat and OS version numbers on the machine.
Ensure that there is no issue with any particular file causing this behavior. Try creating two test files and combining them. See if the files combine or if you get the same result.
You may also refer to the following help document to ensure that you are using the correct steps to combine files: https://adobe.ly/4lbyfBJ.