Help!!What's happening with my shrooms?

[whoopper]

Hi,

I have a rock with aprox. 5 to 6 mushrooms. I have this for like a year and always have look very healthy. First, one the mushrooms starts to shrink and like dissapering on one spot, then the others start with the same behavior. Today only two of them looked healthy and completly open. They are located on the same place and my parameters are stable. Except Nitrates that are on 10 to 20 but they has always been there. I have other Mushrooms on the tank and they are doin great.. What can be happening to this specific rock?? Any advice on what to do?