Stove top coffee, Moka pot question

Had a little 3 cup Moka coffee pot for a few years. Does a great job. Fill the water up to the steam valve, coarse coffee grinds in the little mesh holder. Screw it up and put it on the stove. Just got a 6 cup unit, exactly the same but a bit bigger of course. Doesn't seem like all the water is coming up from underneath? Doing everything the same as I did with the smaller one. Normally the smaller one as soon as you hear it spluttering and spitting it's done. I did that with this one, and realized all the water hadn't come up and gave it more time but that was it. Left 30% of the water in the bottom. Any ideas?