Retain chilli powder taste till end of cooking

When I add chilli powder to an Indian curry, and let it lid-closed for next 5-7 minutes, by the time it’s cooked, it loses it’s taste and the curry becomes bland. What should I do to retain the spice and colour (as even a pinch of Turmeric and 2 spoonful red chilli powder makes the curry yellowish? Why does it become bland??