The reason for conducting the present case study was the problems the researchers encountered during data collection for another research project (Primary Study) entitled "The effects of virtual versus traditional flipped classes on EFL learners' grammar knowledge, self-regulation, and autonomy." Two online questionnaires were administered for the Primary Study. However, the data analysis revealed an inconsistency in the participants' responses which could threaten the validity of the original study. Thus, the researchers decided to design the present case study (Secondary Study) and delve into the reasons which led to invalid responses in the Primary Study. The researchers believed such a study could help future researchers who intend to collect data via online modes. In doing so, they decided to conduct face-to-face, semi-structured interviews. Fifteen participants (six males and nine females) from the Primary Study volunteered for the interviews. Besides, the five members of the administration staff in the Primary Study who had collaborated in data collection were interviewed to give the researchers a clearer understanding of the online data collection. The interviews were recorded, transcribed, and analyzed, resulting in five major themes. The problems of online questionnaire completion obtained from the interviews include sampling bias, low response rates, respondents' lack of motivation, lack of confidence, and lack of researcher/administrator. The data extracted from the interviews also had some highlights on how to improve online data collection. The researchers suggest integrating online and paper-based data-gathering procedures when facing problems in data collection. The present study has implications for researchers who intend to perform small-scale studies. Also, researchers involved in large-scale investigations can find informative opinions regarding administering online questionnaire surveys. [This content is provided in the format of an e-book.]