Based on the discussions and explanations from moderators, I think you are right. The rule is the final nu drops unless the word begins with a vowel or with a KEPT consonant (κ, ξ, π, τ, τσ, ψ). The Gk mnemonic device is κάποτε ψάξε. But they also said that the next version of DL will be more consistent, while this version sometimes retains the nu and sometimes not. If you look to the Related Discussions on the right, you will see the sorts of comments I just made here. I'm just repeating what I learned.