The word lack poses problems similar to shall/should. What we mean by lack depends on the context.
We jokingly may say that someone is lacking social skills because he wipes the food from his mouth onto his shirtsleeve. Here we mean he is deficient in but not devoid of social skills, because if he had no social skills he would be eating shirtless at the dining table. But at least he knew that he had to be dressed to eat with mixed company, so he has some social skills, if not many, and lack means not enough.
When we tell a child that humans lack gills so they can't breathe without assistance underwater like fish do, we mean we have zero gills. In this case lack means not at all.
Let's look at a business example. A supervisor of six operators might email her manager, "I'm lacking staff this morning." The manager would not know whether she is only one staff down (not enough) or six staff down (not at all). The supervisor would do better to write, "I am down two staff."
Finally let's look at the case of receiving a bounced $100 check. The bank might say, "The account lacks funds." In this situation, the check recipient does not if the account has $0.00 or $99.99 because she still gets nothing. The writer of the check would say he had insufficient funds, but the receiver of the check would say she received no funds.
There's a moral here. Don't use lack unless you are sure that your readers would lack a reason to misinterpret your meaning.
