need advice on plants not growing
I live in the San Francisco Bay area, very moderate climate and generally great growing conditions. I read the Square Foot Gardening book and, using 3 previously used raised (elevated) box planters, set them up with Mel's mix using peat moss, vermiculite and a compost from a local nursery containing redwood forest humus, worm castings, poultry manure, bat guano, kelp meal, oyster shell, and dolomite limes.
I then planted, from seeds, 2 kinds of lettuce, broccoli, cabbage, cauliflower, carrots, onions, and some chinese lettuce and cabbage. I water everything twice a week, and the soil appears to remain moist in between. They get great morning sun and shade in the afternoon.
Everything sprouted quickly, but nothing is growing any further. I have lettuce that is 1/2" tall 6 weeks after sprouting.
In the same planters, using regular potting soil, I have previously had great success with lettuce, squash, cucumbers and bell peppers in the summer. This is a little discouraging after getting enthusiastic about the book. Do I need to supplant this with something else, or should I not try from seed. I did not like the idea of transplanting because I would be adding in a certain amount of other soil in with the mix from the root balls.
I would appreciate any help here before I give up and go back to normal soil.
Thank you in advance.