Sometimes stepping outside of a traditional space is the best way to unlock a block. Many students have problems strumming stringed instruments, it's not a natural motion for most. A light bulb went off one day when I was explaining how to hand strum a ukulele to someone who was having a hard time with it. Oddly enough I helped her learn to strum with duct tape.
Lots of folks have trouble keeping their arms from floating around when they are playing. When you strum your arm from shoulder to elbow should be pretty stationary. The part that moves is subtle bends in your elbow and then the biggest part of strumming comes from a gentle wrist/forearm twist. My student was having problems with what I call "floating arms" so I pulled the ukulele out of the equation to help maintain focus and put an angled, small strip of tape on her leg a little longer than the width of her ukulele strings. I told her to make her hand into a loose claw or a "monkey paw" if you will because we were learning to strum in a "Down, Down-Up" pattern. "Mon, Key-Paw" and that's how freakin' random I am.
I told her to trace her middle finger down the line with the monkey paw and then her thumb back up the line not going bigger than the tape. While doing this, she would be seated and had to keep her elbow anchored to her body to keep the "Floating arms" at bay. It worked really well!
Above: Hannah did the brush stroke to work on her muscle memory as her homework and then I drew her a special surprise over the painting.
Another method I use to help with strumming is painting! It's very similar to the tape in that you keep the brush stroke a certain length (which is just slightly larger than the width of the strings on your instrument).
Here's a video that demonstrates this technique.
I've found that everyone learns differently and sometimes the best way to get someone to learn an instrument is to pull it out of the equation! Folks will get so focused on the instrument as an object and forget what their body is doing. When you practice techniques without the instrument, the student becomes more aware of what they are doing with their body.
If you are struggling with your strumming, try doing either of these activities, and I bet the next time you pick an instrument up, it will feel a bit more natural to strum each time!