Your question is hard to answer since you haven't told us which files or how you're trying to "open" them, but it is very likely the problem is at your end since most of the files here have been downloaded at least once and if there were a problem with the file, it likely would have already been reported.
Others have answered similar questions, but it might bear repeating. .cbz files are simply renamed .zip files and .cbr files are simply renamed .rar files. Or they should be. Sometimes people may mistakenly rename them the wrong one. There also used to be a problem with uploading one type of file here (I forget which) so some might have been purposely misnamed to defeat that problem.
Some reader programs will open either regardless of which they're named; other programs might not do this.
It's also possible you have a double extension file.cbz.zip or some such. If you have your computer set to hide extensions of known file types, you will see this as a cbz file but your programs won't think so. They''ll know it's a zip file.
So if you have a problem ....
check to see if you're seeing the full correct file name.
If it says .cbr or .cbz as the only extension, try the other one. If that doesn't work, change it back to what it was and
Try to open the file with a decompression program like winrar or pkunzip.
Look and see if there's an additional compressed file inside it. (I.e., a compressed compressed file -- there's no reason that I know of that this should have been done, but it is true of some files.
whatever you get try to extract the files to individual files.
If what is extracted is a single .cbz .cbr .rar or .zip repeat this process
See if those individual files are corrupt and if you have a full set. You can use many programs to do this including the native file viewing program on your computer (whatever default program the extension .jpg is associated with)
If you wish, you can then recompress all the single files once into a single .zip or .rar file and then rename it .cbz or cbz.