If you have a look at your scorecard for the workout you may see that some of the question circles are yellow. This means that the answer is only considered 'partially correct' (either because you used 'next steps' when answering the question, or you got some intermediate steps incorrect).
If you use a 'next step' or get more than 1 intermediate step incorrect, your mastery progress (indicated by the green bar in the workout) will not increase, even if you get the final answer correct. The idea is that if you are getting intermediate steps incorrect, or you are asking for next steps, you haven't completely mastered that concept yet.
Also, it is worth mentioning that Mathspace has a clever adaptive algorithm that assesses how well you understand a topic and gives you easier or harder questions to help you get to mastery.
When you get a few questions in a row that aren't completely correct (either they were incorrect or partially correct) the questions get easier but the mastery on offer gets smaller so you get a bit more practice at the easier problems before progressing to the harder ones again.