The software needs a certain amount of identifiable features in the areas of the images that overlap. First of all, make sure that you aren't obscuring any existing subtle features of the white walls by over-exposing the images, and that you're not confusing the software with parallax errors.

If you're sure that those aren't the source of the problem, then there apparently really aren't enough usable features in the overlap areas. In some cases you can overcome this by carefully choosing how the images are spaced, possibly along with taking more of them. If you're stuck with a big area of white wall that always results in shots that are featureless along one or both sides, you can try temporarily adding some "features".

Mark spots on the wall with small pieces of black gaffer's tape or similar, at least two marks, one low and one high, on each area of the scene that would otherwise be a featureless overlap. You'll then have to remove these from the image after stitching with something like a "clone" tool found in image editing software (there may be something like that available in the software you're using, otherwise try Gimp). This should be quite easy if you use small marks.