I understand that the 6941 works fine. My question is do you have a 6945 configured in CUCM with the correct MAC address? Usually a status of rejected means that the phone is not the correct type: e.g. a phone is actually model XXXX, but it's configured in CUCM as a YYYY phone model.
Ok,, the fact that you are putting a 6945's MAC address in CUCM as a 6941 is definitely the reason you are getting the rejected status.
You need to remove that phone from CUCM completely. Then re-add it by going to Device>Phone>Add New and choosing the phone type of 6945 instead of 6941. You can create a Phone Template if you'd like, but it's not required. You can just add the phone from Device>Phone.