subjunctive preterit

I don't understand why my answer on a exercise:
"I wish we >would be able< take a nice long vacation, just you, me and Toby. is wrong." but in the sample sentence is correct:
I wish she >would wake up< and smell the coffee.
What is the difference?
The correct answer was:
I wish we >could< take a nice long vacation, just you, me and Toby.

I wish somebody would be able to explain it to me.
BR
Marc