Hunting "stale" or heavily pressured birds
If you're hunting ducks that have been in an area for an extended period (as opposed to migrators) and have been subjected to considerable hunting pressure, not calling may be the best option. These birds are well acquainted with local environs, and they can be extremely wary of aggressive tactics—including calling—to get them to come in.
In the final days of the season, callers have two things working against them: the ducks' accumulation of experience over the hunting season and their increased focus on courtship activities. By late winter, paired birds spend considerable time engaging in pair-bonding activities apart from other ducks. In this situation, aggressive calling is often less effective than it was earlier in the season.
Poor calling skills
It's just a fact: some callers would bag more birds if they didn't try to call. Some hunters' inability to "read" ducks and accurately replicate their calls has saved many a duck's life.
This isn't to say that when these conditions prevail, not calling is always the best option. Rather, these are situations when little or no calling may be best. However, ducks don't always do what we expect. Sometimes they respond exactly contrary to how we think they should react. So how can a caller solve this dilemma?
The best approach is to follow the oldest and best duck-calling tip in the book: let the ducks tell you how they want to be called. Try different approaches, and watch the birds' reactions. If you call and the ducks flare or show uncertainty or suspicion, don't call anymore, or call sparingly. Conversely, if you try not calling and the ducks lose interest and leave, next time try a little persuasion, such as a quick, low-volume comeback call as the birds hit the turning point in their downwind circle.