What's wrong.

Please can anyone suggest what I am doing wrong with my Galathea house plant.
Despite watering, the leaves look dull and dry with brown marks and dark colour under the leaves.

 

My only thoughts are over watering.
Could be pests as well, have a very close look at the underside of the leaves.

 

My immediate thought was over-watering but decided to look it up ...

Why does my Calathea have brown spots? - Planet Houseplant

 

Also look for anything that looks like a small spiders web, a very common house plant pest is Red Spider Mite, you usually need a magnifier to see them but become just visible when they start making a web and are in plague numbers.

For watering in Winter, wait until you see the leaves just starting to droop before doing so, too wet and the roots start to rot away.