Question
Individual leaves on Robellini have turned brown.
Double robellini...one plant is healthy.

South Florida location

Answer
I assume since you live in S. Florida that the palms are in the ground.  I would check the water to make sure they are not getting too much.  The browning sounds like a root and/or crown rot which may be caused by a fungus.  You may try drenching the soil around the plants with Daconil fungicide at a 1% solution in about a gallon of water.  Also check for insects down close to the ground as they may be feeding in the lower part of the plant.  Good luck and let me know of you results.