Question: Sometimes I make whipped cream in my ISI cream whipper and it comes out beautifully fluffy and smooth, and other times when I dispense it, the cream looks "ragged" for lack of a better description. Any ideas what factor accounts for the difference?

Answer: 
I was taught how to whip cream with a handheld mixer and there are a few things to consider

Is the whipped cream being over whipped? When you beat it too hard with a handheld it starts to form ragged peaks

Is the cannister almost empty? Gas charged cannisters often 'sputter' a little bit when over-charged or almost empty.
I don't suppose that it could be a little clogged and just need a cleaning in addition to the possibility of the temperature needing to change? If you happen to have another dispenser, you could always fill and use both the same way to make sure one isn't faulty.