Why will my truck not start?

Why will my truck not start? Ok the truck was running battery went dead we replaced the battery and terminals, truck started back up ran fine for a couple days, then a couple days later it wouldn’t start again so we changed the alternator still would not start so we changed the starter still not starting . when turn the key it just makes a click and thats it, thought maybe a bad starter so replaced it again and still nothing , help can’t figure it out.