I've dropped items for friends. Why are they not appearing on the map?

Solution 1:

I don't know true rate of map refresh, but in my inner feeling it's about 20-30 seconds or a bit more. Otherwise, it will be much more energy eating app.

Most efficient way to make items exchange: one, who will pick items will close application, other will drop all items he want and then only first player login, receiving latest map update and pick dropped items.

Solution 2:

The refresh rate for your environment in Ingress is 30 seconds, as of January 2014. The client does it automatically; normally, no refresh is necessary.

Refresh time is a parameter that is set by the server when you log in, so Niantic could easily change it.

In my experience, the most-common cause for missing items is another player who has snatched them up; otherwise the cause might be a marginal Internet connection. You can try force-sync, but (esp. if your inventory is somewhat full) the sync might fail, too.

The easiest way to quickly see new items lying around is to hack a portal (assuming there is one of course). That triggers an environment update as a side effect, even if the portal is burned out.

Solution 3:

By any chance were you driving just before dropping these items?

Ingress imposes a speed limit on movement; as near as I can tell, it's around 40 mph/65 kph. When you perform any action in Ingress, it's as if there is a circle drawn around that point which grows outward at about 40 mph. If you try to do something outside that circle (by driving to a new location too quickly) Ingress doesn't believe your location. Hacking fails. Bursters miss their target. And dropped items? They just seem to disappear.

I suspect burster energy and disappearing items actually appear on a line drawn from your last location to the current one at the 40 mph circle radius point, but have never been able to prove this by backtracking.