March 2018 got their decision made but Feb 2018 still waiting for test invite. Do you know how their system works?
The system is not First in first out. It is VERY ROUGHLY first in first out, but definitely not down to the month. There are many things that come into play: Security background checks take longer or shorter depending on the countries you lived in and so on.
In a bit of a simplified way: A February 2018 applicant would - under normal circumstances - most likely be fully processed before an October 2018 applicant, but could come before or after a March 2018/April 2018/May 2018 applicant.
As a February 2018 applicant there is nothing wrong with you still being in process. Some made it through already, many haven't. Don't get overly nervous.