My Whirlpool DW does like that either...
If I choose half load, only bottom sprayer and and the uppest one work, and the User's manual says to load ONLY top rack in the lowest position (it may be regulated on two levels).
The latest models instead has got 6th Sens HL, Whirlpool calls it "Multizona", that is you can choose the rack in which you want to load and wash:
1. generally the top rack is used if you have got not many dishes left to wash or if they're not so dirty, there only middle and ceiling sprayers work
2. while if you use the bottom rack it's in order to increase the efficiency of the bottom sprayer power generally with a pot&pan's load with an Intensive cycle, ceiling sprayer works as well.
I've been staing alone for two weeks, because my parents and my sister left home on holiday before me, but I WOULDN'T HAVE EVER WASHED ANYTHING BY HAND! So my little daily load has been collected in the top rack only to be washed oneday-onedayNOT.
BYE
Diomede