Hi
it can be due to many factors:
does the UI use flash or java script ? make sure it is up to date !
now, did you configure well the ddns, the router firewall and the PC's firewall you use to access, too ?
and of course, does the problem arrive with another PC or phone or tablet ? if yes you can discard flash and java and look the rest and don't forget to take also a look at your cam config ! may be an errata or faulty information...
hope it'll help you !
cheers
mich