I've notice that too, it depends of a lots of things, If the guests just arrived, they'll not spend time to buy souvenirs and stuff, even foods and drinks are forgotten.
They'll just go right to their favourite ride and after that maybe reconsider to go to toilet or do some shopping. Vendors quit only if I open the stores too soon, I have to wait until The UI tells me that peoples are starving or thirsty.
As for balloons and Souvenirs....i have to say it is a nightmare. I really don't know how to make guests buy them, It is really random. I hope for a fix to that, beacause all of my staff peoples working in souvenir shops and balloon stores just quit. Nobody like theme. [knockout] And I don't know why.