They go for days without pooing! And I mean days unless you pick her up and set her out of the crate to poo and then put her back on the eggs. Depends on what day your on too. If she is setting tight on the last few days of incubation I would not take her off.
I suspect the word "crate" says a lot about your problem. If you in fact have her confined to a crate, you need to get her out and be sure she moves around until she takes care of business. Close the door, and make her stir about. She won't give up on her eggs. If the crate is in a larger pen, put her feed and water as far from the nest as is reasonable. A hen would not naturally want to deposit droppings close to the nest as that would give away the location to predators. I assure you she will eventually go "poo" but if she is trapped within inches of her nest, it may take place right on the eggs, especially if she is a first timer. Jim