Hangfire is processing only one job at the time

Sorry if it’s know issue, or there is a workaround. I didn’t manage to find something similar.

We have 3 servers in tenant running a batch of recurring jobs. Each server is configured to use 20 worker threads (all servers has 4 virtual cores). After a while we see that is only one job is running at each server at the time.

If we reboot application, HF immediately start to run 20 jobs at the time. What can cause this behavior?