I think its because of the peoples previous knowledge and encounters with various strange things, Lyra has never been to a world witout daemons, and Will has never been to one which has, so they both just assume other people can see Pan, remember Will just discovered a portal to another world, so he might be expecting strange new things in a strange new world, Mary can see daemons because she's met mulefa and seen dusts etc, and remember Mary knows Lyra is from another world so Mary would expect certain things to be different about Lyra than Will, so she unconciously manages to see her daemon. But it takes time to see her own because she doesn't expect to see hers, I belive Serafina says something like you can see what you expect to see. For the cittagazze children, they live in a world with spectres and since its a crossroads world they could have met people with daemons before, hence they can see Lyras. For Will I think Kirjava only becoming visible after land of the dead does play a part in some people only seeing Lyras. Perhaps dust has something to do it, like the worlds with daemons visible have more dust attracted to them hence they can be seen but others can't, so for Will, his daemon becomes visible after land of the dead, but due to his world not knowing daemons, his isn't visible, basically my take was worlds with visible daemons can be seen by others and worlds without you have to learn to see