does a yellow bar that pops up above your message that says "sending..." appear? that's because you're internet's slow if that's the case. or if a red pop up appears that says "could not connect to facebook chat at this time" or "so-and-so is no longer online", than that's just a glitch in the chat thing. and if you're hitting enter, and absolutely nothing happens, you probably have a trojan that's blocking your computer's flash from working correctly.
try downloading malwarebytes anti-malware (it's free, and it works), and run a deep scan. remove any trojans the scan finds, and try again. chat should work better after that.