Refund of IGST paid on export of service

Respected Professionals,

We have applied online for Refund of IGST paid at the time of export of service for one of our client on 31.01.2018 and all relevant documents were submitted to Jurisdictional State GST office (Erstwhile sales tax office) .Subsequently they called for verification of documents. After that department issued  a letter after their satisfaction to the full amount of IGST as refund on 23.03.2018. 

But No refund amount is still not credited, we went to State GST office for clarification, they have no clue why the amount is not credited and they directed to us to Central GST office (erstwhile Service Tax Office)....they are also clue less....we have also raised this issue to grievance portal of GST but they replied in a very passive way to communicate to jurisdictional officer,

Now my request to professional fraternity that whether they have already experienced such situation if so, then would be enriched if they kindly share the course of action they have taken or what should be the way out at present to get this legitimate refund.