I bought some flowers in Lomsak last week, very expensive at Bht 10 each. Transplanted into bigger pots and coconut bedding.

The leaves keep going brown, quite a lot on the bigger ones.

Is it because of too much, or not enough water, anyone, please.

Celosia or cockscomb plant.
Celosia require regular watering but do not overwater as they can tolerate dryer soil better than overly wet soil and too much water can kill the plant. Only water when the top 1-2 inches of soil are dry. Pests and diseases are usually not an issue with this plant. 

Remember these plants are annuals, that's why they are so cheap and they do need plenty of sunlight! 
I used to sell 1000's of these to be used as bedding / border plants in and around parks (UK) during the summer months.

Visual feast of flowers for Bangkokians (nationthailand.com)