Usually, mallards will work in for a long time if they've been hunted a lot. The group of mallards where I hunt will easily take 5-10 passes to convince them to come on down, and even then I gotta land a hen or two to pull the rest of the group down. Now, they've smartened up and they will only circle above the shoreline behind me until a hen gets brave and tries to check the spread. Always try to let the first one or two hens land on or near the spread and the rest should start to pull down on the next pass.
Not telling you to NOT shoot the hens, just that they are pretty useful if you have a reluctant group. If the group doesn't bite- fold the suzies. They're delicious.