We're seeing lots of WARNING log messages such as:
WARNING c.c.h.p.f.c.PeriodicFolderTrigger#run: Queue refused to schedule org.jenkinsci.plugins.workflow.multibranch.WorkflowMultiBranchProject@37177326[foo/bar/jobname]

We're using bitbucket and multibranch pipelines and what seems to be shared by the jobs in the warning log messages is that they are disabled.
Checking the code this log messages comes from com.cloudbees.hudson.plugins.folder.computed.PeriodicFolderTrigger.run() when scheduling a build returns null. Checking that logic there is a check if the job (or parent) is disabled and the nreturn null, which would support what we're seeing.

I'm thinking that the logic is either wrong or the log messages is wrong. It is not the queue that is refusing to schedule a build but it's the job that's disabled. I would suggest first checking if the job is disabled and in that case log that (at debug or info). If not disabled, try to schedule a build and if that fails log a warning messages.