I used to own a Kenmore Ultrawash...
And the Prewash lasts exactly 14 minutes. After that, it drains, and refills for the main wash, the heating element kicks on, and the detergent tray pops open (it's a loud click) and the wash arms start.
I kinda wonder, if you were waiting long enough for it to even open? I mean, that maybe your problem right there. Another thing is Powder, doesn't all just fall out, some of the time it sticks in the cup, and takes about 4-5 minutes, to fully dissolve from the water from the top spray arm.
Set a timer for 35 minutes, and see if it's fully dissolved than. If it's not, you need to switch to detergents, probably best to try a Liquid, or Gel first, and then packets.
TL;DR - Give the machine more time for detergent to dissolve. If it's still not working, switch to Liquid, or Gel detergent.