Switching a partition on a table with one to many relation
You mentioned you added datetime to the PK, but it was not partitioned. You also need to specify the partition scheme when you recreate the PK index so that it is aligned.

Also, note there are restrictions on tables with foreign key relationships. See Transferring Data Efficiently by Using Partition Switching.


  Unfortunately, SQL Server didn't ask anything about index creation on partition or filegroup. I performed that with visual features.   


The GUI is nice for some simple quick tasks but not for partitioning a table with billions of rows. Use T-SQL so that you know exactly what is going on. Large tables are unforgiving.