I agree with kksuns - birds that eat whole seeds like chickens do need some grit but birds that hull theirs seeds do not need grit or gravel at all and it can be very harmful so why give them any?
Also sometimes a bird just feels very safe in their cage and dont want to venture out. Mac and Kiwi love to fly and I keep the door to the flight open a lot when I am home. They fly around but Greengirl (RIP) and Sunshine just dont come out. The door is open, their cage mates are flying around but they never seem to want to go out.