By a week, you mean 7 in-game days or a real life week?
If it's been several in-game days (such as more than 3 or 4) that does seem to be an abnormally long time.
I'd like to ask if you are using any mods at all? Things such as TEdit also count as mods. If they have been used at all on your world, they could have effected the Plant Bulb from spawning in an unknown way.
Then could you upload your world file here? I'd like to look into your world myself to see if there are no bulbs. You can use a site such as Dropbox to upload the file if the upload function here doesn't work.