The number of tasks that can run concurrently in a ssis package is dependent of some parameters.

- the number of processors available

- the value of MaxConcurrentExecutables

- the value of EngineThreads

Make sure that your master package adheres to these parameters.

You can find more information here:

http://blogs.msdn.com/b/sqlperf/archive/2007/05/11/implement-parallel-execution-in-ssis.aspx