Problem with NextTblInDoc

• Dear friends,
  My test document has no reference pages, only master and body pages. There are 5 tables on the body pages.
• The first table is 2 x 2 and in cell 1 the background is magenta (which is what is checked for).
• This cell is found
• Re-entering the while-loop after changing obj to obj.NextCellInTbl lets it run to the end of the table
• This is detected and the script should enter the first cell of the next table for further search.
• The log lists this:

«Table #1»        cell 1 which was found
«Table #1»        cell skipped 
«Table #1»        cell skipped
«Table #1»        last cell in table
«»                next "table"

This is not the complete script, just the relevant parts.

obj = oDoc.FirstTblInDoc.FirstRowInTbl.FirstCellInRow;
while (obj.ObjectValid()) {
  $.writeln("«" + obj.CellRow.RowTbl.UserString + "»");
  if (CompareColour(obj.CellOverrideShading, kReserved, sColour)) {
    break;
  } else {
    obj = obj.NextCellInTbl;
    if (!obj.ObjectValid()) { // end of table ?
      oRow = obj.CellRow;
      oTbl = oRow.RowTbl.NextTblInDoc; // look for next table
      $.writeln("«" + obj.CellRow.RowTbl.UserString + "»");
      if (oTbl.ObjectValid()) {
        obj = oTbl.FirstRowInTbl.FirstCellInRow;
        continue; // restart search for relevant cell
      }
    }
  }
}

Why do I not enter the second table?

And BTW there is no non-scripted method to set the user string...