Beware, a spreadsheet tab should be normalized before any cell operation. Knowing that the ODS format allows storage denormalization (i.e. more than one table cell can be stored in a single XML element), the result of a direct cell adressing operation (like "Sheet1", "D1") is unpredictable as long as the target area has not been pre-processed through getTable or normalizeSheet.

Note that this pre-processing is required once, and not before each cell access.

The following code should work better (the given size for getTable is an example; one should provide the size of a 0,0-based area which covers all the cells that will be addressed.
See getTable and normalizeSheet in the OpenOffice::OODoc::Text manual for details about the spreadsheet addressing logic.