Yeah you shouldnt worry too much. Lexi will have her off days and not poop at all. But by the next day she will. Both my pups will poop twice a day. Nico poops as soon as he steps outside! But once he just didn't go and I was a bit shocked! but I realized we all have off days too, and he just pooped a lil extra the next time I took him out..
Maybe a lil extra water? (just a random suggestion)