Maybe this is really a dialogue issue, but maybe it could be a word-usage issue, too. I tend to not vary the way my characters talk. I tend to write their dialogue and words in the style in which I speak. If I didn’t put dialog tags or some other means of identifying who is speaking, I don’t think the reader would know who’s speaking.
I’ve got to go over my novel for this. In some of my writing, I’ve done things like drop the “g” in words that end in “ing” for some of the characters and have one or two of the characters use incorrect grammar like “ain’t” or “I don’t see nobody out there.” But the novel I’m using for this workshop, the characters are all of similar educational backgrounds. They’re college graduates and wouldn’t be using incorrect grammar.
Then, how do I vary their speech patterns and word usage? Maybe think about their personalities and think of how the words they use and their sentence patterns might go along with their personalities?