One of the difficulties with drafting is that the student must take a 3D item (say, a robot part or a building) and translate it into a 2D medium (paper or computer screen). Some people naturally have good spatial aptitude. The students who aren't catching on probably don't have it--yet. If you put a cylinder, such as a water glass, in front of them, they will draw the top as a circle, not an ellipse. They will need extra help just learning the 3D to 2D concept.
It seems to me that going straight from no knowledge of drafting to a CAD program is too great a leap for most people, anyway. You could start with the basics: have some solid cubes (building blocks, sugar cubes, whatever), and make sure all the students can sketch one cube from the standard perspectives (plan, elevation, isometric, etc.). Then have them sketch arrangements of 2-3 cubes. Using colored blocks and pencils may make it easier for students to see the spatial relationships. Then, they can do more complicated arrangements, then sketch things like Lego models, or work with pictures from books.
Their lines don't have to be straight, and their lettering doesn't have to be neat. The point is to develop their ability to see how their 2D sketches relate to 3D reality; that relationship has to be accurate. (If the red block is on top of the blue block, it must be that way in the sketch, too.) They'll need close to 90 degree angles in the plan and elevation views, and highly flattened angles (60 & 120 degrees) in isometric view. Make sure they have graph paper and isometric paper so they can focus on sketching, not getting their lines and angles straight.
Once the students' spatial reasoning has been established, then they are probably ready to learn the conventions of drafting, both on paper and CAD.
I had my son go through a curriculum called DIME 3-D Sketching Project, from Spectrum Educational Supplies, which did what I have described above, and it gave him a lot of confidence. There are three books, which are written for ages late elementary on up. It would cost a bit of money to get the books, which are reproducible, and the special set of blocks. Go to
Spectrum
There is also a book call Practical Drafting, which is designed for students ages 12 and up to use on their own. It's an overview of many different drafting applications (way beyond sketching simple blocks). Go to
Insight Technical Education
This particular web site has a brief sales pitch for the benefits of learning drafting, in case you have some reluctant learners who don't see those benefits.
As far as boring the students, you can avoid doing that if you sometimes let them draft real-life things they like or need to draw, such as robot parts, model cars, even clothing patterns. Just insist that the drawings accurately reflect the real thing, though you may need to simplify. This should be a fun course, and I hope you and your students enjoy it.