I've noticed it takes awhile. After a couple of days, maybe even a few days, mine kicked in, and now I have to keep moving new plants and re-planting. I'm sick of flowers now! I just want to get the last two kinds for the museum and then let them all DIE!
Make sure you're not near the limit for flowers, or you won't get any new ones no matter what you do. You can check the number of plants you have in the mayor's office under statistics. I can't remember the limit, was it 500? Anyone remember? I've been selling all my first generation flowers, I'm down to about 350.