If you know that students are having trouble with a particular topic, I would offer to host an extra information session, but get them to bring along their questions and specify which parts of the topic they are having trouble with. That way you can understand the exact issues and perhaps put a plan in place to address them, rather than having to cover the whole topic which may be quite broad. You could also see if some of the students that are not struggling would like to buddy up with the ones who are struggling and offer their support.

Was the topic expected to be tricky? If so this can cause us to subconsciously expect a certain level of confusion from the students.
I would try to identify real life examples/applications for the topic. Put it into context for the students, this may provide the motivation required to stick with it and gain more understanding.
If they are motivated to find a solution then I would be motivated to help explain it in as many different ways as I can. Sometimes there is a certain level of repetition needed before something becomes clear.
Consider the cognitive load of the topic. Does it need to be broken down into smaller steps?
Does there need to be a focus on the final outcome to provide motivation to get there. (Ruth)

I think both the points that MHubert and Ruth made are interesting. I am a great believer in active learning and asking questions especially when topics are cognitively challenging. Questions generally require answers and therefore provide an opportunity for reflection to see if the student can answer the question. If not, unanswered questions provide the oportunity for the teacher to provide further explanation or for the student to research and find the answer. Providing an extra information session or tutorial provides the opportunity to recap and further explain and research the tricky topic. It also provides the teacher with the opportunity to see if any supplementary resources such as readings or videos need to be provided and also the chance to ask relevant questions to help guide students as well as answering questions the students have.

My go to plan would be based on evidence based practices that would help identify scope and severity of issue.

My plan to identify issues would include: 
1. Active retrieval/free recall on all related criteria (content, order, and time – much easier to use if a visual guide is present)
2. Rapid assessment of learning (slava kalyuga protocol) – full mentalization process, steps they write down tell me the limit
3. Assess in desirable difficulty stages
4. Identification of human error type (mistake, lapse, violations). Mistakes mean they misunderstand or do not know a rule (misconceptions fall into this category also), lapse means they know but failed in execution, violation means they know the rule but intentionally disregarded it.

My design plan would be:
1. Have a pre-learning test (prior knowledge measurement)
2. Measure cognitive load during pre, during, and after learning (build in via self report, timed events that can be automatically captured)
2. Have visuals to support learning
3. Have question types grouped and labelled by difficulty and by principles learnt (both singular and similar)
4. Have a plan of materials that can go backwards if learners fail (returns back to last concept but has different surface details).