Eufy e20 output not consistent

I recently got an eufy e20 and have been testing it out. I use a medela symphony as my main pump and get about 4 oz during my day pumps. The first time I tried the eufy I also got 4 oz and was happy because I felt like I couldn’t go out before and everything revolved around my pumping schedule. The second time I used it I only got 1 oz.

What could be the reason? Nipples weren’t aligned? I was walking around instead of sitting?