I have a similar issue. I just set up a new project https://www.inaturalist.org/projects/primates-of-malaysia?tab=observations but I cannot add certain observations by other observers or by me that would qualify. The project won’t pop up in the project list on the right task bar. All the observations that are already listed in the project now were added automatically.
From the Managing Projects page:
Observations are never “in” a collection project; they either meet the project’s requirements and are displayed when the project page is loaded, or they don’t. You cannot add or delete individual observations from a collection project. However, note that you can always edit the project to tweak its settings.
Can you provide an example of an observation you think should be in the project that isn’t showing up?
One thing to note, that unless the submitter follows the project, the fact that the observation is collected into the project will not appear in the ‘projects’ section of the observation record. That only happens if the user follows the project.