I had lots of issues with trying to run parralel review workflows (different tasks approvals for different depts), and was only able to get this working with any efficiency by using the Start Workflow action in the following Parallel structure - meaning this workflow would not complete until every branch Start Workflow had started properly.
(Before this, I had all the tasks in one giant workflow, in parallel. It was a nightmare adding pauses and loops). I am not sure if this is applicable in your case. But it may be worth a try.