This post needs a little more information to troubleshoot:
1. What user licence do you have?
2. In the settings of the webmap is it enabled for use in Collector?
3. Are you creating your webmap from services hosted in AGOL or Enterprise? If enterprise, what version are you running?
4. Are there any services in there not supported by collector? Such as a feature layer generated from an uploaded shapefile?
5. And the biggie, is there at least 1 editable feature service in there? Collector and field maps won't show maps without one.
6. Finally, try accessing the web map with Field Maps.
David
..Maps with no limits..