Solomon’s Boneyard – Is There Any Benefit to Honoring All the Graves?

Ok!

So, it seems that they updated I don't know when but they did. You just have to be patient (3-4 days in my case).

Problem solved!

As Matthew says in the comment, there are sometimes bugs either with games or the Game Center. (I posted a similar question about missing achievements in Cut the Rope.) Unfortunately there is probably not anything you can do about this besides reporting the issue, which you can do via PvZ's entry in the App Store.

The only other thing I can suggest is double-check that you do indeed have everything - there should be 49 plants in all, including the Imitator. And try playing one level using the Imitator, just to make sure it's registered. (The same might go with other plants, if there is one you never planted during the course of the game.)