Thanks for getting in touch! I have a couple of ideas here. First thing to check is the task cap on your Octopus server. Octopus should be able to perform 5 tasks simultaneously without any issues by default. The following documentation page has instructions on how you can adjust the Task Cap to your preferences.
Changing the task cap may be all you need to solve your issue. However, you could also look into is our new Spaces feature. Spaces essentially let you partition your Octopus installation up into separate working environments.
Let me know if either of these options help or if you have any further questions.