This is most defiinltey due to have a Signing Order where you must wait for the other signers to add their signature, fill any other required fields, and Finalize before you are able to sign it. When you send a Sign Request, you have the option between signing “all at once” or “one at a time” in the window before sending. The article below explains more:
How do I send my PDF as a Sign Request?
You’ll see a label for Waiting for Others next to your name and email address at the top of the active Sign Request when the other signers need to Finalize before you.
You will receive a notification email once the other signers before you have Finalized and when it’s your turn to sign.