Indexed mode is reducing image to only 2 colors
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I created a new palette in gimp using my original RGB image (containing shades of yellow and black) which gave me about ~200 colors (with interval=2 in Import dialog). 
Then I changed the mode from RGB to Indexed using the palette I created above, but the resulting image contains only 2 colors - black and yellow. All the dark-greys, light-greys and different yellows are absent.

I want my indexed image to resemble the RGB as much as possible but the result looks disastrous. Please help. 

Visual illustration of my problem : http://i49.tinypic.com/w1tchw.png