Hi Kamila,
without more information it will be hard to suggest where to look. Where are you calculating this, are you using a Tax solution from the Solution Exchange? Where is the calculation defined and how?
Analyzing the data on a parent member may also not be the best place to look as bookings on base accounts may revert each other, for example.
It usually helps to write parts of the calculation result into the error log to see where it may go wrong.
Generally, more information would be great for the people in the forum to help you.