How old is your snake? It is quite normal for a snake not to poop regularly. You say your snake is losing weight. How often are you feeding this snake two mice? It may be time to bump up the prey size since you are feeding two mice. I have a snake that is a bit on the small side for it's age and I am now going to feed it more then one mouse at a feeding. This snake weighs 285 grams and is very slowly gaining weight for me. I would not be to concerned about the not pooping often.