There are lots of times when a particular course does not fit and your grade can suffer. If this is a rare occurrence it will not likely affect your graduate school application seriously, unless that particular course is a mandatory prerequisite for the degree. If you retake the course and show improvement, you may be fine.
An example situation affecting applicants is doing poorly in an advanced course that most people do not take. Is getting a C or C- in Calculus III better or worse than not taking Calculus III at all? Even seemingly easy courses like Acting for non-majors could be bad for your GPA if you were really a mismatch. When you have a few single courses with low grades and your overall grade trend is normal or competitive, the analysis comes down to three key questions:
- How important are those classes?
- Why did you not perform well?
- If they are important for the program, how can you compensate for that weak preparation?
Courses that are important to future success in or for qualification to enter a masters program logically get more scrutiny from the admissions committee. If your weak courses were not in your major, not related to the masters degree curriculum, or are unimportant for the committee’s decision, then they pose less admissions risk. If the courses under scrutiny are in your major or prerequisites for the degree program, the admissions committee will be very concerned about your performance.
If you have an opportunity to retake an important course, you should consider it carefully. Retaking courses allows you to achieve improved results, strengthen your GPA, and shows the admissions committee that you are determined. Of course, the only acceptable outcome when repeating a course is to show improvement and significant improvement is ideal.