Why might I have trouble making butter from Crème Fraîche? I tried making cultured butter today from a supermarket variety crème fraîche. For some reason the butter hasn't split from the whey although I've "churned" it with the hand mixer for 20 minutes or more.

This recipe for cultured butter suggests that about 2 to 5 minutes is needed with a food processor, and my previous experience with the hand mixer is about 10 minutes for sweet butter.

I checked the ingredients list of the crème fraîche and I don't see anything unusual: cream* and an unspecified culture. That's it. No artificial thickeners for example.

What could be the problem?

Update:

I'll just note one avenue I've already explored: temperature. I've tried varying the temperature during churning from just above freezing to around room temperature. 

* The cream is "högpastöriserad" which is Swedish for a pasteurization process where the cream is held at 80°C for 5 seconds. I'm not sure this term translates directly to a common English term.
If you were using supermarket cream, the reason would be very clear: it is not only pasteurized, but also homogenized. This means that it has been mechanically emulsified to prevent the butter from separating. 

I don't know if the cream for creme fraiche is homogenized too before creme fraiche is made, but with your result, I'd suspect homogenization too. It won't show up on the label as an ingredient, as it is done by forcing the cream through microfilters.