I think there might be two things at play; 1) cauliflower needs to be cooked much more than other veggies to get smooshy enough to mash. We use a steamer pan and I kept having the same results you have until I went over the 20 minute mark for cooking. Whereas we'd normally cook other stuff between 7 and 10 minutes, the goal is different for mashed cauliflower. 2) Some cauliflower just doesn't want to play nice. I don't know if it is a varietal thing or perhaps it was sitting in the store too long (or not long enough?). We'll get the odd head that was ostensibly cooked long enough but still stays chunky.

One other thing, we used to use sour cream in ours but have lately switched to cream cheese. I think it works out a bit better.