hmmm... never had that happen to me. It's worked under every test environment, every platform, every browser all perfectly (that's kinda the point of Java you know?)
Anyways, maybe try a few clearing of cache or a "CTR+F5" on the browser to force it to reload again. What browser/OS/Java version are you running? If that's still not working maybe try and go
here to download the latest version of Jumploader. Once you've downloaded the file, just upload "jumploader_z.jar" (must be named that name to work) to the directory phpg/Javascripts/ and maybe that will fix things. Otherwise, maybe try posting this question on the Jumploader support forum
here.