This message has been posted a while but just in case someone else has a similar problem here is a fix.
It has nothing to do with the sword being badly made all swords come loose. I've fenced for about 10 years so I have some experience with swords (If you've ever been to a fencing competition you'll see people shaking their swords checking for rattles and then running around looking for allen keys to tighten up them up.)
To fix it all you have to do is take off the pommel (the metal bit at the end) usually held on with a normal nut. Once that's off you'll see the tang (the metal bit that runs through the handle) that usually has a normal nut tightened against the handle. Tighten it and reassemble, problem solved.
What I did on mine to make sure it won't come loose again (or come loose quite so quick again) was I put on a locking nut. This is basically an identical nut tightened up against the first and then the one closest to the handle is loosened a little.
This should fix the problem but if your going to use a sword it's going to get loose just through the stresses of using it.
Hope it works