Moving columns in Numbers - it's not letting me do it!

I seem to have this bizarre bug in Numbers, and it can't be normal behaviour. If I click on a column tab, it doesn't highlight the one column, it highlights the whole spreadsheet. Similarly, I can't select, say, cell D1 and drag the cursor down to D20 to select a column that way, because after highlighting so many cells, it will go and select the whole sheet.

I've searched the net for a solution and haven't found one, and the only thing I suspect is causing the problem is some merged cells. Problem is, I can't split the merged cells back into individuals, as after five or six attempts at doing it, it adds extra cells and messes up the spreadsheet (one I had to import as an Excel 2003 file since FAME doesn't do Numbers or CSV).

Any ideas? I'm stumped.