Java doesn't detect new version of applet when deploying from the same URL

Question

If have a webapp that loads an applet from an URL. Whenever I deploy a new version of the applet (accesible through the same URL), the clients don't detect the change and work with the cached version.

Why doesn't Java / browser plugin detect new verions of applets?

What alternatives do I have? (e.g. adding headers like entity tags, change jnlp, etc.)

This is my jnlp:

<?xml version="1.0" encoding="UTF-8"?>
<jnlp spec="1.0+" href="LITE_afirma.jnlp">
 <information>
   <title>My package</title>
   <vendor>My vendor</vendor>
   <homepage href="http://www.my.url" />
   <description>My description</description>
 </information>
 <offline-allowed/>
 <security>
   <all-permissions/>
 </security>
 <resources>
   <property name="jnlp.packEnabled" value="true"/>
   <property name="jnlp.versionEnabled" value="true"/>
   <j2se version="1.6+" href="http://java.sun.com/products/autodl/j2se" />
   <jar href="my.jar" main="true" version="3.2.1"/>
 </resources>
 <applet-desc
   name="My applet"
   main-class="com.my.company.package.MyApplet"
   width="1"
   height="1" />
 <update check="background"/>
</jnlp>