The below article covers the number one reason why scheduled posts are not auto-filling in a schedule.

Default Accounts for content don't match the selected accounts in the schedule

This is one of the most common reasons why queues aren't pulling in content. The selected accounts in your schedule must be at least equally matching to the selected default publishing accounts on the content items.

The category tags match for my schedule and content library yet my queue is still empty. 

Check the upper right of your schedule to view which accounts are selected. Then check that the content also has at minimum the accounts selected within the schedule.

To update the default accounts on your content. Go the Content Library, find a piece of content that should be in your schedule and click the three dots in the upper right corner.

Under the Publishing tab, select the appropriate default accounts corresponding to the accounts in your schedule.

Click Update.

Content should now auto-fill into your schedule!

You can also change the default accounts of multiple pieces of content at once with Bulk Operations. To do this:

In the Content Library, click the gear icon in the center of your screen. Select Bulk Operations, click Select Default Accounts then Select Content

You can now individually select your content by clicking each item or you can select an entire category from the left side of the screen.

Click Add Items

Select the appropriate default accounts then click Add Default Account

All your content should now auto-fill into your schedule!