Co-author keeps delaying crucial part of paper
I have been on both sides of that situation, so I know how frustrated you must feel.

However, the reality is that the number of "collaborative" research that fizzle out and never get finished due to lack of interest, willingness to actually do something, or time is a lot more than it should be. I could give you multiple stories from my experiences but I won't bore you with the details. Let's just say that there are over a hundred hours of my time sleeping in various people's desks and email accounts that will never come to fruition.

Your collaborator may simply be busy, or the worst case scenario is that he isn't able to actually deliver on what he promised and is stalling because they are embarrassed and/or don't want to admit to it (I've had this happen). Regardless of the reason, this is not a good situation, for both you and your collaborator.

Face-to-face and/or heart-to-heart conversations have resolved such situations (in one way or another) for me, but I can't say that it works for everyone. Still, it wouldn't hurt to try. Just let them understand you simply want to know whether the project is still alive or it's time for you to either find other ways to get the project finished or simply move on.