Thanks for the replies, guys, these are informative
I do own the entire Duchies of Crimea and Cherson; however, I am playing Project Balance, and (apparently) Taurica was replaced by the "Kingdom of Crimea", which consists of nothing more than the Duchy of Crimea (now that I think about it, this may simply be a bug, seeing as I have no idea why this would be done), while the Duchy of Cherson has no de jure Kingdom. Do you think this may be why it isn't drifting?
On a related note, I own several other pieces of land that I would expect to be drifting, but aren't. I inherited the Kingdom of Denmark peacefully (well, relatively peacefully) and own the whole thing, but it isn't currently being incorporated into Russia from the de jure Scandinavia. Another example is the Duchy of Azov, which, again, I own the whole thing, yet isn't being incorporated from Cumania (which is a Kingdom, it has no de jure Empire).
Edit: Oh, I almost forgot to mention that the Byzantines never owned Crimea, historically. Is the Kingdom of Taurica de jure Byzantine? Or just Cherson?
Yes, if your duchy of Cherson isn't de jure part of a kingdom, that would probably explain it. I don't have any experience with that myself, though, seeing as I play vanilla; and there every duchy is de jure part of a kingdom. The way the mechanic works (in vanilla) is that an entire duchy can only drift from one kingdom to another. If under 'Project Balance', your duchy of Cherson isn't part of any kingdom, that would probably explain why no drift occurs.
Edit:
OHHHHH WAIT. I feel really stupid now. Okay, I understand why Cherson isn't being incorporated into my lands. Cherson is part of the Kingdom of Trebizond in Project Balance. I don't own the Kingdom of Trebizond yet. God I can't believe I didn't notice that sooner -.-
I could be mistaken, but if you control the entire duchy of Cherson (which you do) and the kingdom of Trebizond exists and is not held by you, the duchy should drift towards one of your kingdoms, though. :-/ Unless the drifting mechanics have been changed from vanilla as well.
Are you absolutely sure
all counties and duchies that are a de jure part of the kingdom of Denmark are under the control of that kingdom? This might differ from 'Project Balance', but in vanilla there are a couple of provinces in what is now northern Germany (real world) that are considered a de jure part of the kingdom of Denmark but are de facto under the control of a Duke in the HRE (Oldenburg, Lübeck etc.). You can view these here:
http://ckiiwiki.com/List_of_Kingdoms
If you control the entire kingdom of Denmark (and all counties and duchies which are a de jure part of it), then I don't see why it wouldn't drift towards you. Unless it has something to do with the empire of Scandinavia not having been created: it's not impossible that the kingdom (of Denmark) doesn't drift towards you if the title from which it should drift (empire of Scandinavia) doesn't exist.
In vanilla, the duchy of Azov is a de jure part of the kingdom of Alania (together with the duchy of Alania). As the kingdom of Alania is already being incorporated into the empire of Russia, that might explain things - though, again, vanilla might differ from 'Project Balance'. De jure drift always occurs towards your primary title, though I'm not exactly sure how this works if you hold an empire (Russia) and several kingdoms under it.
If you look at the map I linked, the entire kingdom of Taurica, including the duchies of Cherson (and Crimea) are considered de jure part of the ERE in the vanilla game.
From recollection, the Byzantine Empire historically seems to have held several 'possessions' on the southern coast of Crimea until at least 1204 (though the precise nature of these holdings is open to debate: they mostly seem to have been vassal states and/or protectorates), so the decision is not
entirely unfounded. I do agree that it doesn't make much sense to include the whole of Crimea and Cherson, though, but the problem is the way this mechanic works: both Crimea and Cherson (duchies) are considered a de jure part of the kingdom of Taurica, which in turn is a de jure part of the ERE (in vanilla). It would have made much more sense to make the southern strip of the Crimea into a separate duchy and kingdom, but they probably steered away from this course because of gameplay reasons.
There are several other instances where this mechanic creates ahistorical situations - Flanders, as I mentioned, being one of them.