It happened to me also while trading EURUSD. And it's not that I receive any message that my orders were rejected ... no ... the platform just "plays dead"
it behaves as if I haven't touched anything. Sometimes the "equity" value from the bottom left corner also freezes.
However, I have observed in time that if I use the Web platform, this didn't happen. So my best guess is that it has to do with the JForex platform, more specificaly with the way it handles communication with the live feed and trading servers.