Students can do poorly. There might only be one, perhaps even none; the Holy Grail. Perhaps a few, quite a few. All struggling students should be a concern at the start. None should be dismissed, discounted. In a student-centric teaching environment, those floundering are not ignored or barely supported; they are one of the issues to focus upon (e.g., Pedersen and Liu 2003).
Once you know why a student (or students) is not doing well, you can figure out what to do. We do not believe that there is valid reason to do nothing; not attempt something to help the student(s). Until you do the root cause analysis, the source of the problem and/or the remedy might not be in the student's domain!
There are students who start off poorly and there are students who start off strong and then tank. There are likely different things going on.
So, it is not an easy job being an instructor and taking a professional, responsible perspective. Dealing with students performing poorly can require different skills, knowledge, and methods. It takes time and effort to learn how to monitor for grades or mental health issues, and how to address any suspected issues.
You need to have clarity about the cause and effect. Is the problem with performance student based, course/instructor based, or a combination of the two? If there is course/instructor component, it is important to address it if possible, through a course change, or through another mechanism that compensates (e.g., extra help from the TA). It should never be assumed out-of-hand that any poor performance is student based, even if you are getting 10-15% A's, and if there are many other students doing well. They might be doing well in spite of you, not because of you. There might be Type I, II errors going on, and correlation does not mean causality.
If the performance across students seems linked in time, and if the students have been doing ok so far, it possibly has something to do with the instructor's end of the business.
Depending on what you find in the analysis, you might find that your assumptions about what the students know and what they should be capable of are flawed. This is possible and if it is the case, adjust the course accordingly.
If many students are involved and the issue can be tracked to one key concept or lecture, do some additional intel and see if there was something going on in that lecture (or outside of the lecture) that caused the gap in learning and understanding. There might have been another big assessment due, other demands on the student body. It might not have been your class, but your class was collateral damage.