Beginner problem with table not splitting over 2 pages

I'm trying to print a table that will always split over two pages. But 
when I set the row heights to add up to anything more than one page 
length, what I get is the bottom half of the table overprinting the top 
half, not splitting onto a new page as I expect.

Splitting tables has always worked for me before, but the last time I 
wrote a ReportLab program from scratch was ages ago.

Any suggestions on what I am doing wrong?