In part b) if you put in the answer as the same as the question then it is marked as incorrect and an exclamation mark appears, but no message.
In parts f) and i) if you do the same thing i.e. answer the same as the question then this is marked as correct.
I cannot see how this difference of treatment of the answers occurs and what generates the empty message.
I am looking at Martin Jones' question https://numbas.mathcentre.ac.uk/question/1875/powers/
All are mathematical expression parts.
In part b) if you put in the answer as the same as the question then it is marked as incorrect and an exclamation mark appears, but no message.
In parts f) and i) if you do the same thing i.e. answer the same as the question then this is marked as correct. I cannot see how this difference of treatment of the answers occurs and what generates the empty message.