Like the title says Brook, Robin, Franky, and Ussop should have learned about a started to develop their haki between the end of Zou and before Luffy and the others get to Wano for Robin, Franky, and Ussop, and after WCI for Brook on the way to Wano.
We know there was about a little over a month maybe a month and a half between the end of Zou and Luffy getting to Wano. In that time is when they should have been training.
Off screen training is not a bad thing as long as it makes sense for the characters, after Luffy, Sanji, and Brook see first hand the true power of a Yonko and knowing they are going to confront another one, it would make more sense for them to train, Luffy even does with his advanced CoO.
As for Robin, Franky, and Ussop they are in Wano preparing, that should have included training along side their jobs recruiting and getting closer to the Shogun. Usopp awakens his Haki in Dressrosa and doesn't wonder at all about it?
We know there was about a little over a month maybe a month and a half between the end of Zou and Luffy getting to Wano. In that time is when they should have been training.
Off screen training is not a bad thing as long as it makes sense for the characters, after Luffy, Sanji, and Brook see first hand the true power of a Yonko and knowing they are going to confront another one, it would make more sense for them to train, Luffy even does with his advanced CoO.
As for Robin, Franky, and Ussop they are in Wano preparing, that should have included training along side their jobs recruiting and getting closer to the Shogun. Usopp awakens his Haki in Dressrosa and doesn't wonder at all about it?