The critical path is the series of dependent tasks which, if any of their durations change, will cause the whole project's duration to change by pushing out the last task in the project.
Not having your project in front of me, here's a few things you can check in your project:
Make sure your tasks are connected by dependency connections. If they're not, a task that's delayed by a little bit may not delay your project end date and won't be considered a part of the critical path.
If a chain of dependent tasks have some gap of time between one another that can break the critical path. The reason for this is because if there's a gap of time, that means the task technically has some wiggle room before it starts to affect the project's duration. Once you close that gap of time the critical path will be visible again. How do you close up the gaps in time? That involves the task's scheduling setting which I'll explain below.
By default, all newly created tasks are set to start "As early as constraints allow". This is why new tasks automatically start at beginning of your project and not on the current day. Now, if you use your mouse and drag a task in the gantt chart to set a new start date or if you enter a start date manually into the start date field of the task, the scheduling is changed to start "On specified date". It's anchored to that start date and won't show up on the critical path that task won't move automatically anymore regardless of dependency connections, leveling, etc... You want to make sure all your tasks are set to start "As early as constraints allow" for it to be part of the critical path. If you have a task that has been set to start "On specified date" you can click on the "reset" button in the Task : Information inspector.
Does this help? If you're still seeing issues with the critical path in your project, you can email us your OmniPlan file to firstname.lastname@example.org
and we'll take a closer look.