The five biggest things I've seen that make IPMI's Java applets not work.
1. Mac OSX (no clue why.. Macs just don't want to work right with the virtual media feature).
2. Java isn't up to date.
3. Java has some customized settings that were added to make some other software (often proprietary software) work properly.
4. You're connected to the IPMI through a wireless link somewhere.
5. Connecting through something that is creating latency. AKA going across a VPN tunnel, accessing a server that is physically across the country, etc.
Hope this helps.