Can you share your data? Because anyway unless the roll engine is faulty the past won’t tell you about the future rolls.

You really can’t fix it by artificially nerffing because assuming that the platform has random enough dice (which is most likely does for any real needs) the fact that they have thrown poorly before doesn’t mean that they will do so after your changes.

I don’t know how you are keeping track of the rolls but if you aren’t, I would first try that to truly see if it really is the case that the rolls are lower than average. Our memories of things going poorly aren’t objective and tend to enlarge amount of bad outcomes.