Undeveloped patches (appearing as brown / bronze / white patches ) is caused by damaged film. This could be caused by accidental pressure applied on the films before they were exposed or mishandling of films.
The problem usually tends to lie with old film that is expired and not stored properly.
DO NOT BUY FILM FROM BIG W, I REPEAT, DO NOT BUY FILM FROM BIG W!!!
They store expired film on shelves where they are subjected to a broad range of heat and humidity conditions, which severely impacts the quality of the film.
You can try this method too: