It's common for Facebook remarketing campaigns to struggle to acquire a large enough audience in order to run. Facebook says their minimum audience size for a pixel-generated audience is 1000.
This means the Facebook pixel on your site needs to be able to match at least 1000 website visitors to Facebook users.
What factors into this?
Match Rate - Facebook doesn't reveal their average match rate (the rate at which website visitors can typically be matched up with Facebook users), and this likely varies greatly depending on the type of user who visits the site (do they tend to have Facebook accounts? Do they tend to reset their cache & cookies often?) as well as a number of other factors.
Pixel Age - The pixel is only able to see website visitors in real time, which means it can't see people who visited before the pixel was added to the site. So, if you've had the pixel on the site for 6 months then it will have 6 months worth of website visitors to try to mach to Facebook users (as long as the settings inside the Facebook ad account are set to look back 6 months, which is the maximum amount of time Facebook allows a pixel to look back).
So, the best thing to do is get the Facebook pixel onto the site early, make sure the settings inside FB pixel area are set to look back 6 months, and consider sending more people to the website with proactive (aka not remarketing) ad campaigns in order to populate that Facebook remarketing pixel audience.