All depends on how stable the track is.
I know all of mine work online as I do AI & online test rooms before I release them to the public.
Although they're modded we still have some rules to follow to keep the track stable, some people just make really long tracks or highly modified tracks & release it without testing it.
Sometimes it takes a long time for the track to load up for some people depending on there location & internet speed or if PSN is having one of it's funny days or not, as they have to download the track data as they enter your room.