jstack: Target process not responding

Question

I am running Ubuntu server edition and I wanted to take a thread dump of Tomcat.

So, I first tried to find out which PID tomcat uses:

$ jps -l
5809 sun.tools.jps.Jps

But it's not there?

So, I used top instead and found out the PID 5730.

Then I called jstack to get the thread dump:

$ sudo jstack -l 5730
5730: Unable to open socket file: target process not responding or HotSpot VM not loaded
The -F option can be used when the target process is not responding

What's going on? :-(

I already tried to export CATALINA_TMPDIR as described in Jstack and Jstat stopped working with upgrade to JDK6u23 but that didn't change anything:

$ export CATALINA_TMPDIR=/tmp
$ sudo /etc/init.d/tomcat6 restart
 * Stopping Tomcat servlet engine tomcat6
   ...done.
 * Starting Tomcat servlet engine tomcat6
   ...done.
$ sudo jstack -l 5934 // new PID after restart
5934: Unable to open socket file: target process not responding or HotSpot VM not loaded
The -F option can be used when the target process is not responding

Update:

I also tried sudo -u tomcat6 jstack -l -F 5730 > threaddumpexceptions2.txt but it only gives me tons of exceptions on the console.