Credit Karma is pretty good for what it is, but it sounds like you need to go to the source(s). If you aren't aware, you are entitled to one free credit report per credit bureau per year. Take advantage of that and be sure to get the report from all 3 bureaus individually (ignore any ads or whatnot that want to charge you a fee for it - it's totally free). Go to
Annual Credit Report's website and you can order them there from each of the bureaus.
Review each of them very carefully and look for errors and you should be able to see any negative remarks that are hurting your score. One bit of advice, if (when) you find errors, don't challenge them on their websites. Handwrite letters and send them to the credit bureau via registered mail. They have 30 days to respond, and if they don't, they have to remove the negative information. There should be many examples on the Googles about the format of the letters.