Thanks Vitaly Furman

We enabled logging on receiving PACS and figured out that Ensemble was sending same ID for 2 different SOP classes. I had been using the "CreateAssociation" command with third argument as blank to create associations.  When doing so , Ensemble adds all Abstract Syntaxes it knows about with default TransferSyntax of Implicit VR. I believe the receiving PACS didn't cater for one of the Abstract Syntax. 

To solve the problem, I removed the association. Created it using GUI so no default Presentation COntext is added. I then manually added a few Abstract Syntax with Basic Transfer syntax of Implicit VR and retested and this time the communication worked.

I then created an Import file for the PACS based on Conformation Statement provided by PACS and imported in Ensmeble using the ImportAssociation command.