Christine,
I apologize if this is what your workflow is already looks like but I recreated a workflow with my own data to try to replicate the issue. This is what I found to be a successful workflow.
If you have published as a feature service as a hosted feature layer and in the hosted layer Overview page and enabled attachments, make sure to check your hosted feature Settings section “What kind of editing is allowed?” and enable at minimum “Add and update features”.
Create a WebMap, with or without features. My test data included three (3) points for bird sightings. One (1) of which already included an attachment from when I published as service in ArcMap 10.6.1 to AGOL.
After creating a WebMap I opened collector on an iOS platform. When in collector, I selected an existing feature then selected the feature description at the bottom of the viewing window to pull up the feature details. At the top of the details page I select the Options button (looks like an export button), this allows for edits to be made. Selected Edit, then selected attachments, then attached an item to the feature. Below is the screen shot workflow.
Once the feature has an attachment it should look like the below image.
Let me know if this helps.