Performance Document approval routing issue

In the performance document approval process: when the requestor ie manager submits for 2nd level approval, it is not routing including not generating the email notification also for ONLY couple of employees BUT successfully routing for most of the employees.
What could be the reason?What is the resolution?We know the work around: we recovered or reassigned to the Manager/requestor and asked him to resubmit.But we want to know the exact root cause of the issue as we want to take precaution for the issue.

High level Steps to receate:
…………….
Logged in as Requestor
-          Resubmitted for approval: Jimena Gonzalez