If your tasks are marked as running, but they aren't actually running, then they won't be run again. You can clear the running status by editing the task, disabling it and then re-enabling it - no need to delete the task and re-add it.
As to why they don't run - I am assuming it is because they are stuck in the running state?
As @brodkin says, one task taking a long time can effect the other tasks - we need to revisit this to see how we can improve it.
As a workaround, you can run tasks directly from the console by calling