Differences in Lava Lamp Activity

I have two lava lamps that rest in one of the window sils in my classroom. It is both a great distractor to the students and a source of fascination/curiosity to my students. It also helps that my students have to learn the concept of density. One of my lava lamps always seems lethargic and to be frank quite a dissapointment for viewing. I had been thinking that maybe it's a really good observation that might lead to a science inquiry for my students. I did some video and time lapse filming which may result in an inquiry for my students to do. Take a look at the video and see if it's a good idea.