You can only gain a card that is at that time on top of a pile. In your situation, there were 3 Elders on top, so even after gaining one there is still no Miller on top of any pile.

I used your game as a basis for another, where there was 1 Miller at the top, and 1 Elder underneath (game 119210478 at the end). Here you can see that you can gain Bazaar, Miller and Elder.

Note that the order of clicking is important here. Since the Miller is currently on top in the pile, you must choose (click on) Miller before Elder. If you do it the other way around, you will only gain the Miller (+ Bazaar).