"Why did they increase the price?"
Translation:Dlaczego zwiększyli cenę?
I think that's just not a standard collocation. In theory it seems okay, but to me it sounds as if they increased the size of the font used to write the price.
"zwiększyć" also isn't that great, but surely better. I checked the corpus (300M segments), I found 30 results for "zwiększyć cenę" (includes all correct forms) and 0 for "powiększyć cenę". But there are also 97 results for "podwyższyć cenę" and the winner is definitely "podnieść cenę" with 526 results. Those two are accepted, of course, but they are not taught in the course.