One of my cleaner shrimp just die

...checked the water and every thing was fine...ammonia at 0, nitrite at 0, phosphate at 0.25 ph at 8.3 salinity is ok, nitrate at almost 0....all the water perimeter is fine/.....what do you guys think is wrong with this because my other cleaner shrimp is ok...had them for about a month now.....all the fishes are ok....the two anemone seem to be doin ok...everything in the tank is still alive except for the one shrimp....they are beeing feed good because one just sucessfully molted..