It's certainly OK to ask open-ended questions after a set of closed-response questions, but in your specific case your question might have been a bit too abrupt to encourage responses. Many of your respondents might not have known the answer, or the answer might have been complex. If you had asked in a slightly different way, perhaps you would have received more answers. Furthermore, there might have been different reasons for failure in different semesters, and participants might have considered it too burdensome to go into details.

I wonder whether, with a bit of piloting and/or careful thought you could have asked for the information that was of importance to you in another closed-response question.

Obtaining data is never incorrect. However, the type of analysis of the close-ended items and of the open-ended one has to be different.

Your open-ended question is suitable for a more qualitative approach in which representativity is not such an important issue.

In the worst case, if you feel that the answers to that question are not worthy, you can just discard them and focus only on the close-ended questions.

There is no problem in asking open-ended questions as it improves the interpretation of your close-ended questions.

Your respondents merely exercised the option not to answer the open-ended question. Use the answers you have gained to rank the cause of failure.

Yes, you may want to asked optional open-ended questions for further elaboration and comments on the issues. This flexibility of investigation will allow the respondents to recognize the issues of failing in English course. As a result, the scores between the two sets of data would be slightly different from each other and definitely would emphasize more on the quantitative part for the convenience of your respondents. Integrated the results into the interpretation to support or oppose your outcome.

It's certainly OK to ask open-ended questions after a set of closed-response questions, but in your specific case your question might have been a bit too abrupt to encourage responses. Many of your respondents might not have known the answer, or the answer might have been complex. If you had asked in a slightly different way, perhaps you would have received more answers. Furthermore, there might have been different reasons for failure in different semesters, and participants might have considered it too burdensome to go into details.

I wonder whether, with a bit of piloting and/or careful thought you could have asked for the information that was of importance to you in another closed-response question.

Thank very much indeed for your valuable responses. @Antonio M. Oller-Marcén, @Nivea Thomas, @Mark Trevor Marais, @Bahram Shahedi and @Robert Trevethan.

I would like to add here that some points:

1- Some students do not like to respond to questionnaire which take long time and if I made the open-ended question obligatory, they would respond with any information to allow them submit the responses.

2- I asked this question to elicit more responses from those who want to say more about their failures factors.

3- I distributed the questionnaire to only failure students who failed English in the previous semesters and were studying it again at the time of data collection.

Jamal Kaid Mohammed Ali . You're very welcome, Jamal. I guess that it's often difficult knowing how best to ask questions to get at the issues we want to investigate. You're not alone!

I hope you can get some useful insights from your research.

One approach you might consider is to determine whether there were systematic differences in who did or did not answer the question. You can do this by creating a 1-0 variable for replied or did not, and then using that in a series of t-Tests (or chi-sqs) with the other variables in your study.

Of course it was---and the response that you got was----just what it was.

Of course it's not a problem to ask open questions after a series of closed questions, but, your question is very sudden to encourage your respondent's response. Many of your respondents may not know the answer, or don't want to answer. If you ask in a slightly different way, maybe you will receive more answers. There must be different reasons for failure in different semesters, and participants might find it too burdensome to go into details. with a little trial and / or careful thinking you should be able to request information that is important to you in other closed response questions.