As Andre said, Mount Requests are caused by not having enough media in the media pool that can be written to. In over 17 years supporting Omniback/Data Prtoector, I have never seen a change in schedule cause a Mount Request
In your backup specification, go to the Destination tab. Expand the checked Tape Library. Right-click on a drive, and pick Properties. Make a note of what media pool is selected, even if it says <No Media Pool Selected>
Go to Devices and Media, find the same Tape Library, and expand it. Expand Drives. Right-click on the same drive you checked in the Backup specification, and get Properties. Click on the Advanced tab, and see what the Default Media Pool is, which is what DP would be using if you saw <No Media Pool Selected>
Go back to your Library, and single-click on the word 'Slots', and you will see all the storage slots displayed. Click on the column header 'Pool' to sort the media by media pool. Find the Media pool being used by this backup. You must have some media there whose condition is Green (Good) and whose Protection is either None or Expired
If you heve Barcode support enabled, do a Barcode Scan to bring the IDB up to date