Perhaps you can share the agent(s) you are expecting it to run on, and the job configuration of the jobs that aren't scheduling?
If the job isn't being assigned, it means there is no free agent[1] in the desired pipeline-scope environment[2] that offers the requested job-scope resources [3]. All three conditions need to be met, so either your agent or the pipeline/job may be configured in a different way to your agents (perhaps it is requesting a "resource")?
Keep in mind that you don't have to use job resource requirements or environments if you don't want to, and you can configure it such that some agents will be able to run jobs with no specific requested resource; or no specific environment.
-Chad