I recently built a raised bed waist high to grow lettuce mixes outside the back door. It gets afternoon sun. I've seeded it a couple of times now, and the seeds sprout very quickly, and then immediately go to seed. Don't even put out leaves, just a spindly flower stock. I used Miracle Gro potting soil I think. It's odd, I've got a patch of arugula that has self seeded in the sandy soil nearby, full sun, and it's lush as could be (luckily I have somewhere to pick fresh greens!) Any ideas why this is happening, I've had raised salad beds before at another house and never had a problem.