Some servers take much longer than others for daily backup

Hello

Some servers take much longer than others for daily backup. 

All servers have about the same amount of data and yet some servers need 12 hours daily to backup the data and the rest only 20 minutes.

Can I somehow find out why this is?

Then I have a 2nd problem:

On a few servers the daily backup does not start automatically. I have to start this manually every day. I get this error message:

The backup did not start as scheduled. Either it was skipped automatically because the previous backup was still running, or a backup scheduler error had occurred.

How can I fix this?

Regards
Michael