Been thinking a bit about this.
Try to recapitulate what you did and how you did every step from loading the film, through exposure etc. until they were developed.
We don't mind if you "jot it down" into an answer here, so that we can help you pinpoint the problem.
One thing, as you say that every negative looks the same. Are they exactly the same? Did you at any point, e.g. while unloading the cassettes, stack them up before loading them into whatever you use for developing? Etc...
//Björn
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