Newbie question - triangle never turns green?

[LeisureTime]

Hi folks, like I said newbie here, so be kind.

Just learned from the wonderful video yesterday how to lock a target using "L" key.

I'm doing the Quick Mission "Ambush in the Palawan Passage" and get by the destroyers, surface and lock on the battleships anywhere from 400 to 1200 yards and the triangle thingy turns from orangish to bright yellow but never turns green.

Am I missing a step?

Thanks in advance!