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It almost looks like vignetting + insufficient overlap are issues here - your pano software or its settings might be off too. Also, some "pieces" look like they were exposed differently. Were you exposing manually or using a priority mode?
I don't do panoramic shots too much but to get seamless blending, I found it is really important to sufficiently overlap the images and expose manually and consistently. I think armando's estimate of is about right but I personally lean towards a 50% overlap to be safe.