Providing affordance for round knob style range sliders

I have an application that uses round knob style range sliders like these:

You can see an example of this slider here.

To adjust the range - the user clicks or touches the slider, and drags directly up or down.

Now - I believe I have a good reason for using these sliders. The application is primarily targeted at mobile users, the the round style gives a larger target area for thick thumbs. Also - the round style allows me to use less height, at the cost of more width.

However - what I've noticed is that users will often try do a circular interaction with the slider, and really struggle with it.

Is there a way I can improve the sliders such that the user knows how to use them correctly?