Can tables from Pages be reused in Numbers to create graphs?
I have had the best luck keeping the data in Numbers and only pasting tables and graphs into Pages. But that is because I like the added tools in Numbers and not that I've had any issues pasting back or forth.

I've done a few tests and can't replicate your issue on '09 versions of both iWork programs. Have you experimented pasting the data from pages to numbers as values (or paste and match style)? It is a long shot, but the two things that come to mind is pasting into a new spreadsheet (in case the one you are using has some sort of issue) or pasting data that is somehow formatted to cause an obscure problem.