Onboarding journey inconsistently allocating to initiator or no line manager

We have set up an onboarding journey for a specific group of employees (approx. 60 employees) which were manually allocated and have noticed a number of inconsistencies / issues as follows:

1. Journey is inconsistently allocating to no manager (i.e. no journey or tasks allocated for manager) when it should allocate to the line manager as seen in the system. There are <20 employees impacted where journey was not assigned to their line manager when there is a manager in the system. How do we allocate the checklist to the manager? Noting that the new joiner has already started their checklist tasks. Can we avoid deleting and reallocating?