Re: My data in my Model is not being added correctly
Yeah, echoing the other comment that we'd need to know the formula as we're shooting in the dark here.
Though, as a general answer
1) Summing Line Items: Best Practice is to keep simple sums AS simple sums, and this can be checked through the blueprint, the next column after Formula is Parent, and those should get filled-in with your Ending Balance Line Item if they should sum to it.
2) Sum of List Items: if that's a list, one check is of course the Summary setting (is it possible that's on average?)
And in general, check for line or list items that may have been hidden.