It's still early in the season! Organic compost shouldn't throw your ph or nutrient levels off insane scales though, it's a very slow release, and if that Were the case, I believe the leaves would develop chlorosis and start to look sickly at the bottom of the plant. It could be your spacing.. When I have two too close together, one will just grow right on top and over the other. Battle of the fittest! But like I mentioned, it's still early! I may have two that are starting to form heads now and probably 70 that aren't. As for the cauliflower, are you in the south by chance? I heard it doesn't tend to do well there.. I've got broccoli and mine all seem to be growing at different rates also, even though they're all the same age.. Weird but I'm gonna let nature do her thing! How is your bee and insect population looking? That may have some sort of impact on certain developments.