Jenkins detects SVN changes but doesn't update workspace?

Are the clocks on your Jenkins and SVN server in sync? If the clocks are off by even a little, strange things will happen. I do not use SVN much myself, so I do not remember if this was one of the symptoms but it would be good to start by checking the clocks.

Easiest would be to install ntpd on both hosts, configure them to sync against the same source and forget about it. If you do not have an ntp server in your organization, check http://www.pool.ntp.org