Long distance collaboration with a professor – wrote manuscript, getting ignored. What to do next?
Since email isn't working, I see two alternatives. The simple one is to contact the person who originally got you involved to see if they know what the problem might be. Presumably this person knows both of you and may know the other person better than you do. 

The only other solution with a positive outcome is to contact the other party in person. Your question is "How shall we proceed?" not "What is the problem here?". There might be any number of things in the way, but you have no way to  know with a passive approach. 

It's not impossible that you are being used and therefore ignored. But you shouldn't conclude that without more information.