I made a new webmap for Collector today, but the option to allow for use in Collector is missing in the settings. It is still visible in the older maps I created earlier this summer. My boss is doing the Field Maps beta and can see the maps in his Field Maps, but not Collector as well. Is this a bug or did something change on how you access maps in Collector in the last month?
The first thing I would check is that the new map contains an editable layer. And if it does, but that layer was recently made editable, try saving your map again.
+1 for missing editable layer. Fastest way to check is to see if there's an 'Edit' button in the Web Map Viewer.
Here's a couple of techical articles that go into more depth and provide additional solutions:
Problem: ArcGIS Collector does not display web maps created in Portal for ArcGIS
Problem: Unable to view a web map in Collector for ArcGIS
I had the same issue, where a new web map with new editable layers was created, I was unable to open the map within Collector.
I found that once i manually created some random data in the new layers it appeared in Collector.