This may be due to the following condition in /_pages/open_projects.md:
{% if project.status %}
{% continue %}
Whereas many of the projects listed in /_data/openprojectlist.yml do not include the status parameter, and hence are not listed on the Open Projects page.
If this is by design, then this issue can be closed.
Otherwise, please let me know the statuses (ongoing/completed) of the projects listed in openprojectlist.yml