Could cheese "halt" the tenderness of cooking lentils? I was cooking some lentils in chicken broth. When the lentils seemed almost done (they had been simmering for 30 minutes or so), on a whim, I added a fair amount of Gouda cheese. There was still plenty of broth left, the cheese barely thickened the liquid and the taste was nice. I was looking forward to the lentils being done so I could enjoy them.

It's been over an hour since I added the cheese. I've kept it at a simmer ever since but the lentils are still not done. If anything, they are tougher now than when I added the cheese. 

Is there a reason cheese would have this effect? There is still plenty of liquid, I started with at least 4:1, and there is still at least a good half inch of liquid above the level of the lentils.

What's going on?

EDIT From this question: Can Calcium Chloride be Used to Prevent Lentils from Bursting? I gather that the calcium in the cheese may be the issue, but it seems that it shouldn't have that much of an effect.
I have always been told that high acid-yielding foods can cause dried legumes to stay hard. I can't say that I have ever added Gouda (although it sounds delectable) but I have had dried beans stay hard when adding tomatoes too early in the cooking process. Wondering about this, I looked up Gouda and was quite surprised to find that it is a high acid-yielding food. 

Could be other reasons such as legumes that are old or a hard water situation but I suspect you have already considered and eliminated those as you are very thorough. :)