Problem with data storage. How can I look inside the "black box"?

[LisWinter]

Dear all,
I am very new to the "programmer world". What are the possibilities to find out what went wrong with data storage for a couple of participants, while it had worked perfectly for others?
I ran a jspsych experiment (Stroop-like experiment). I sent the link to the experiment via mailing list to the participants and it worked perfectly for 63 subjects. But for 8 subjects the experimental data was not stored. The jspsych code was on the university server and there the data was saved after the experiment was finished. (I think) I know that data loss occurred because participants provide personal data after the end of the experiment to receive credit for participation. This personal data is stored in an extra csv file separate from the experimental data. The csv file with the personal data was stored on the server for (I think) all 71 subjects, but not the csv file with the experimental data. (There are 71 csv files with credit information but only 63 csv files with experimental data). Now I want to run 3 follow up experiments that are very similar.

How can I find out what or where it went wrong in these 8 cases? I cannot replicate the problem. I have tried everything I could think of to create the problem (using different browsers, ending full screen mode during the experiment, strange response behavior). I am really lost because I don't know what I can do or which tools I can use to look inside this “black box”. Can I possibly use the next experiment to figure out the problem?

Thanks in advance

(I'm not writing the code in here because it's long and there are three custom plugins.... And the error could be anywhere, I think. But if it would help, I will be happy to post it😊)