Parallized Stored Procedure is executed synchroniously

Hi,

we use the azure data factory with a foreach loop to do calculations with a stored procedure. This used to work very well but for no reason the running time went up from some minutes to some hours and sometimes we get error messages like this:

Execution fail against sql server. Please contact SQL Server team if you need further support. Sql error number: 10928. Error Message: Resource ID : 1. The request limit for the database is 400 and has been reached. See 'https://docs.microsoft.com/azure/azure-sql/database/resource-limits-logical-server' for assistance.

After some investigation in the sql server we recognized, that the 50 queries that should be run in parallel seem to ran synchoniously in azure SQL. There is always only one query running. All others are suspended. 

What can be a reason for this error and behavior? 

Thanks and regards
Holger

Regards
Holger