Availing cenvat credit

Sirs..

We are a manufacturing sector, we are getting an agency commission from a German company.

We are paying service tax on the amount received, based on reverse charge mechanism (since 2010) and claiming this as Cenvat credit.

We came across 2 audits, no one questioned this.

But recently in an audit, they have raised an issue stating that this is not eligible for availing cenvat and they insisting us to reverse the credit taken since mar 2013. 

I need your opinion in this regard. Thank is advance.