Secondly, it's inaccurate. Everyone has a voice. Everyone. Every writer who ever wrote anything writes in their own unique style, which is what "voice" apparently means, first and foremost, these days. So what critics mean when they say your writing lacks "voice", then, is that it lacks an individual style that separates it out from everyone else's writing. Okay, now that we've cleared that up, what can we do about it? And is it realistic to expect writing to carry a truly "unique" voice that doesn't contain echoes of other voices you've read before? I say this because, realistically, no one writes in a vacuum. We are all influenced by one another, and every writer I read reminds me of someone else I've read. Every single one. None of us have ever recreated the wheel, not entirely. It's unrealistic to hold a writer to that standard.
First, playing devil's advocate, let me ask, why do we need an individual voice? What's so bad about sounding like the author you love? Why shouldn't you aspire to copy, or mimic their style? When you take beginning writing classes, that's exactly what you do; you learn how to imitate the style of the writer you're reading, so that you can prove to your teacher that you understand the basics. The idea(l) that lies beyond that level of writing skill, however, is what plagues most writers. The ideal is that you will not want to mimic someone else, and that it is bad to do so.
The problem I have with this goes back to what I was saying above; it is entirely unrealistic to aspire to what critics love to label "a brand new voice." That's hype. There is no such thing as a brand new voice. What there can be is a new take on an old subject, told from a different perspective, using language you're not used to hearing. Anything reminiscent of someone else's writing that sounds too much like them, though, will be trash-canned, either for real or on someone's desktop.
Why is that? Why is the search for that "new" or "fresh" voice so important to the writing community? I believe some of it stems from the perpetuated belief that the act of writing is really akin to the divine. There is that unexpressed hope that you're going to read something you've never seen before that will lift you out of the mundane. That seems to be a deeply-rooted need, a wish-fulfillment fantasy for readers of all sorts. We're taught so many things about writing that allow these myths to persist, that the myth of the "fresh" voice is just one of many.
If voice is about your style, though, about how you say the same old thing, that's different, and that can be taught, and it can be nurtured. It is attainable. It is no longer a divine mystery.