There are a lot of possibilities here. Firstly answer this, what is incorrect and how is it incorrect?
Some of the first thoughts are double check your filters, double check the raw data is up to date, double check your calculations. I've not seen tableau miscalculate since the ages of tableau 7 and even then it was extremely minor. That's not to say you have not found something thats broken.
If you provide a sample of the data you are using (or the packaged workbook itself), explain what value you are expecting and why, and show the value that's currently being returned we can go from there.
Thanks carl for your suggestion