Ah, right -- we've got some odd behavior there as well; newly-shared folders not showing up in particular.
I suspect there are bugs in how the (locally-cached) LastPass vault stays up to date, possibly coupled with a delay on the server side for updates to be propagated to all vaults.
Just from my experience working with encryption... I'll bet the sharing process is fiddly, and they have some unresolved flow problems. E.g., imagine a new folder share offer is encrypted and queued for the recipient's vault, and can't be merged in until the recipient next signs in... but depending on their browser plugin settings, they may only sign in once every few weeks. So the plugin pings for queued updates and can send up auth keys... but already we're getting into enough complexity that bugs would be likely.
(I don't actually know how their folder sharing functions, but deploying encryption where the server only occasionally has the keys can quickly make simple requirements turn into Rube Goldburg contraptions...).
In our case, the shared folders eventually showed up for everyone (we specifically tried signing out of LastPass and restarting browsers... though I don't think we quite nailed down an "always works" solution), and once the basic shared folders were set up, it was smoother sailing.