It's not so much the volley shot itself, but the getting to net that is difficult. S&V has a reputation for being a very difficult way to win (even in the days of "old-style" racquets/strings/courts, etc) and it is. It just takes way longer to get good enough at it so you can win that way. I lost a LOT at first (I just ignored the bagels and kept at it - lol) when I was learning to S&V and have found it much more difficult to learn than winning from the baseline. I also try to get to net as often as possible during a match besides serves.
The hard part, which Marius alluded to, is the transition to the net from the serve itself or from approach shots. Your serve has to be good enough to come in on, with faster serves not necessarily better. It takes much, much practise to develop the kind of court sense to be able to immediately seize the right opportunities to come in on other than serves. I find that, even though my volleys and approach shots are technically sound, I make more errors because I hesitate and come in just a split second too late or get over-excited and come in when I should have maybe hit a couple more balls for a better opportunity to approach. Also, it is hard (at least for me) to transition from a hard groundstroke or overhead, for example, to a much more touch/finesse shot up at the net without missing a beat.
Practise obviously, but also perhaps change your expectations about the time involved in the learning curve because, I have found, that S&V is about a lot more than just the strokes themselves. It feels awesome to win by S&Ving so be patient, keep at it and good luck!