I would add that if your walls are plain and with no visible texture or features, Photoscan will have trouble with that. The program needs to be able to identify features so that it can discern depth and placement of elements in its final model. Blank white walls are IMHO one of the worst things to try to scan.
You can project a texture onto the wall. I've seen that done with models also. Project a random texture onto the walls to give Photoscan something to see and grab onto.
You might ask how to get the proper colour of the walls back in your final texture. I think the answer to that is to also shoot the walls with no texture projected onto them and then simply switch out the textures. You'd have to set your camera up and project the texture and shoot and image, then turn the projection off and shoot an image, move your camera, turn the projection and shoot an image, turn it off and shoot an image and so on. A bit tedious but that's what I would try and do. Name the untextured images and the textured images the same but put them into different folders. Build your model with the textured images. Switch them out before you go to make your texture.
Maybe someone else here has a better idea but that's my 2 pence.
Jon