[whpalmer4]

A couple of things to watch for:

Actions with a due date will have that due date overridden by the project due date if the project due date is earlier. In such a case, the actions you are expecting to see in the "Due in the next month" group might be filed in an earlier group, corresponding to the due date of the containing project or action group.

Actions in a sequential project (or sequential action group) won't show up if they are not available or due soon. If you have a sequential project with an action due tomorrow and a following action due in 10 days, you won't see the one due in 10 days because it is not available until you've completed the previous action. You would see it if it is 6 days away, because that is inside your Due Soon limit, and so OmniFocus would display it even though it is not available (a matter of some controversy).

The Due perspective I believe uses the Due Soon action selector by default, which means you aren't going to see actions from more than a week out with your settings. Turn on the View Bar, change the selector to Remaining actions and see if you suddenly get that Due in the next month group.

I believe that one of these cases will explain what you are (not) seeing. I haven't had any issues with the Due in the next month group not working properly.