I recently bought a Vivitar 90/2.5 Macro Nikon AI mount to use with my NEX. I bought a Fotga Nikon-NEX adapter from eBay. I mounted the lens but now cannot unmount the adapter I know Nikon turns clockwise to unmount. I am rotating it clockwise while pushing the little tab on the side of the adapter downwards, doesn't seem to do anything. Is there anything else that I am missing?