What MazeCraft about it being too dark can be correct. If you're planting it underground be sure that every crop has a little bit of light near it, if it's above ground, light should not be a problem. I planted a little farm underground and the lack of light ended up being the problem for the seeds popping out.
What I do myself is place water under the walls of the room, which avoids any exposed water (a nuisance if you or crops fall in; you can use lily pads but it is easy to accidentally mine them); for lighting I use glowstone in the ceiling, as shown below, which gives you an idea of the lighting requirements (I've also used torches placed 2 blocks up and/or recessed in holes in the ceiling):
Note that the light level of the block directly above the crop must be 9 or higher, or 8 at your feet (as shown on the F3 screen, assuming you are playing 1.8; older versions show the light at head level so you can directly use what it shows).