I tend to agree with d3's sentiments. It happened to me last year (small local races (the only type in which I am competitive) where I couldn't open up on another guy, but I never felt that he was, to use a derogatory biking term, "wheel-sucking." In the final stretch, I stumbled a bit, he passed me, and I couldn't close on him.
I don't think there was anything wrong with it, although I might have if I thought he was just toying with me. I decided to try to open up on him rather than slow down and force him to take the lead, so that was my call. Afterwards, he said he thought I had him with a half to go, and so did I. I didn't. But I blamed myself. I was upset, but not at him. At me, and vowed that it wouldn't happen again.