You submitted a paper, not a draft of the paper. Reviewers have no responsibility to turn your marginally acceptable paper into an accepted paper. Their responsibility is entirely to the editorial staff to make a recommendation as to whether the journal should publish the paper.
You don’t say what the tone of the reviews you got back were, but if the referees see a real problem in the “easy” part that would preclude publication, and they have problems understanding your “complicated” part, no, they don’t owe you a review of the complicated part.
Where I tend to see reviews that don’t address many of the issues in a paper is when there are real basic problems that will have a clear impact on the paper as a whole — in other words, when the paper needs some real work before referees can really offer a comprehensive review.