Why are some questions in my quiz not working?

I would appreciate it if someone could have a look at a four-question extract of my quiz.

Two of the questions work perfectly well and the other to respond with a message 'you must complete the question before submitting'. I can't find any difference between the key aspects of the definition of the parameters for all the questions.

My detail score is not high so it may be something simple! But any help appreciated.

John.a.