Just how close to a planet do you have to be to transport stuff down there? I've run into this problem both on the Romulan and Gorn campaign and have been as close to 10k trying to fulfill an objective by transporting things to the planet.

Or am I missing something else?

5.9k is the transporter range for most items.

5.5k is the transporter range for marines on hit-and-run raids or capture attempts.

With lag, the AI can seem to beam from 8k in combat.