servlet <session-timeout> ignored

Using the servlet configuration parameter <session-timeout> and setting it to larger than 5 minutes fails to work. The session is timing out before five minutes are up. This is a major problem as my users need to be able to work on a page for longer than 5 minutes.

I have had to make my own session timer and Ajax requests to keep the session alive in order to make my site work. This used to work but after upgrading to 1.5.7 and later it has not worked.