If you want to use DB1 / SL please make sure your have Accounting Periods set up way in the future.
DB1/SL usually starts with DB depreciation ana then changes to SL ... at the point in time when the SL depreciation gets higher than the DB depreciation.
If you still don't get the depreciation calculated as expected after you checked the Accounting Periods then please add some more details, like acc. acquisition costs (value and dates), depreciation (values and dates), DB%. No. of Periods etc.
Or gather the mentionened information and reach out to your partner and ask the partner to check and/or raise a service request at Microsoft.
