The most common reason for unscheduled orders are Prod. Order Routing Lines in the production order that have 0 setup time and 0 run time, which means there is nothing for MxAPS to schedule, and in the case of serial routings, that also prevents any later operations from scheduling.
You may also have enough production orders or large enough production orders to require more time on a specific work center or machine center than is available. When you generate a schedule the default “Schedule Duration” is set at 30 days, but depending how you create production orders you could have many months worth of production orders. You can set a scheduling filter to reduce the number of production orders you are scheduling or increase the “Schedule Duration” on the “Generate a Schedule” page to 90 days or longer. Remember that the longer your schedule duration the more data the system has to process and write back to BC, so that can slow things down. You may also have incorrect or very long setup or run times, time units of measure, or a large order quantity that will take more time than the capacity available, which can also cause unscheduled orders.
Finally, if you have configured secondary constraints you may have component items or other constraint items that are unavailable which can prevent operations that require those items from scheduling.
Our product support can provide more specific advice on using the troubleshooting functionality to create a scheduling request file that can then be analyzed by a support team member.