Just to back up a second. Whether a task is critical or not is a mathematical calculation in Project. if a task has Total Slack >0, it is not critical. (Total slack in Project is = float in the PMI terminology world). If you add the Total Slack field to a table, do you see a value greater than zero? If so, by Project's definition it is not critical - while it may be very important to you.

You can alter the definition of critical in File > Options, Advanced and you can increase the value above zero.