Backup issue

We have 1.2 tb DB. every day full back and every half hour log backup was configured. Today full backup got failed. Generaly full backup was taking more than 4 hrs. So we planned to configured Diffrential backup instead of full backup and every week -full backup and as usual log backup for every 1/2 hr.

When i executed the compressed diffrential backup. Using query i got to know that its taking more than 17 hrs to complete the backup. So after 2 hrs we stopped the differential backup.

As todays full backup was not their we started the full backup then using the query i got to know that same like previous it was showing 4hr time to complete fulll backup..

What could be the reason differential backup was taking more than 17hrs.
when i stopped and executed the full backup asusua its taking 4 hrs time. what is the reason for long time ?
Anything mistake i am doing?

Until i started diffrential backups We have succeeded fullbackups in drive.
Todays fullbackup got failured due to some issue but later on i started diff which was taking lot of time. So i stopped this also..
Please help what causing differentail long time. After differential stopping if i run the fullbackup again its taking normal 4 hrs time?

Thanks in advance.