try this
1) open gimp (duh...)
2) select the colorpicker tool
3) in the tool options window it will show the color pickers options
4) at the bottom right of the tool options window their will be a reset tool options button it is a blue circle with a yellow arrow pointing around it, click this
5) now try again and see what happens

edit: i was able to duplicate what was happening to u by changing an option when i clicked thsi option it wouldnt set the color, so reseting tool should fix it because the option was in tool options