A lot of people are complaining about this. I was one of the first to try placing the authorship tag and over a period of time it has been switched on and off several times. Sometimes my authorship stats appear in the lab section of webmaster tools.
It has been suggested on this forum that Google do a manual review of your site before authorship gets approval.
I think that it is going to take time and is still probably in the experimental stage.
My website was pandalised yet my ugly headshot still appears.
Do many people have you in their circles? My 'real' Google+ account is in about 500 circles, and my picture was showing up within a couple days of adding authorship markup.
Conversely, a friend of mine has a site with roughly the same authority as mine, but is in only a few circles, and his photo doesn't show.
I can't say the circle count is definitively why this is happening, but there could be something there. I can say that I have never noticed seeing a photo if the author is in only a few circles (the stats are displayed next to the image in the SERPs).
Having your picture show up in the SERPs is secondary. Are users more likely to go to your page if you're good-looking? --if you look like the stereotype of an authority in the field? (Cooking site: Use a picture of your mother. Medical site: Use a picture of your grandfather. Legal site: Use a picture of Bruce the shark*. Et cetera.)
It's about, well, authorship. When someone swipes your material and then swears they wrote it first and it was you who stole from them-- that's when you point to the Verified Author line.
I'm with you, Lucy24; it's why I don't use my own picture. Don't nobody need to see THAT mess. But I do want to establish authorship.
I am in a gazillion circles; a couple thousand I think. Fortunately for my own site, it doesn't matter all that much. Still, I'd like to know it's right so I can apply it to other places more important.
I tried Google+ to see if it was for me and they grabbed my mug from an older blogger site profile and used it on my behalf. I changed that picture to one of a skull xray of me and a month later they started using that. I don't use G+ anymore but still see that skull in many places.