Lower the bar to attract more contributions
I think we can agree that getting more contributors involved in the Open Source project is generally considered a good thing.
In order to get more people involved in your projects you need to keep working on reducing friction and lowering the bar for newcomers to enter.
Remove or minimize formalities necessary for contributing code or other content to your project. Be friendly and assist newcomers by smoothing over rough edges or obvious obstacles that can become blockers for the success of this user's mission. And if manual assistance is needed, take that as an action item and make sure that little bump in the road is ironed out in time for the next new contributor. Make sure the tools and recipes work as documented, with as little extra hands-on as possible.
Long response times is also a sort of bar that can seriously slow down contributors. Try to make at least the first reply quickly. And as mentioned elsewhere, add tools and bots to unload humans from tasks that can be automated.
For every new contributor you manage to "onboard" into your project, the more members you have in your project who can then help bringing in the next wave.