It looks like it's drying out too much. Indoors the air is usually pretty dry which means you need to be attentive with watering. That said, if it's kept too wet you can get root rot which shows up the same way as being too dry. When roots start to rot it restricts the plant's ability to take up water.

Thank you for the reply. The fronds have completely detached on the right plant. Is there anything I can do?

Sorry, I've been away. If you can increase the air humidity around it that would help. That's not easily done if you live in a dry climate. You can put plastic around it but you need to keep the plastic from touching the plant.