Считывается только часть ячеек excel interop, остальные null

Попробуй сделать так:

        public DataModel GetDataFromExcelFile(string filePath, int startRow, int startColumn, int endColumn)
      {
        Excel.Workbook wb = null;
        Excel.Application excel = null;
        Excel.Worksheet xlSheets = null;
        try
        {
            excel = new Excel.Application();
            wb = excel.Workbooks.Open(filePath);
            xlSheets = wb.ActiveSheet;
            Excel.Range range = xlSheets.UsedRange;
            int rowsCount = range.Rows.Count;
            var startCell = (Excel.Range)xlSheets.Cells[startRow, startColumn];
            var endCell = new object();
            endCell = (Excel.Range)xlSheets.Cells[rowsCount, endColumn];
            var writeRange = xlSheets.get_Range(startCell, endCell);
            var data = writeRange.get_Value();
            var dataModel = new DataModel
            {
                RowCount = rowsCount,
                Data = data
            };
            return dataModel;
        }
        catch (Exception ex)
        {
            return null;
        }
        finally
        {
            wb?.Close(false, filePath);
            if (xlSheets != null)
                Marshal.ReleaseComObject(xlSheets);
            if (excel != null)
                Marshal.ReleaseComObject(excel);
            if (wb != null)
                Marshal.ReleaseComObject(wb);
        }
      }