Final answer:
Images in a concave mirror may not focus properly due to varying angles of light incidence and irregularities on the mirror's surface; using small radius mirrors helps mitigate this issue.
Step-by-step explanation:
Not every image always forms on focus in a concave mirror primarily due to varying angles of incidence and irregularities in the mirror's surface. When light rays strike the mirror at different angles, they reflect differently, which can result in images forming at locations other than the focal point. Additionally, if the mirror's surface is not perfectly smooth or has defects, it can cause images to be blurry or disjointed because the reflection of light is distorted. Moreover, spherical mirrors may not have a well-defined focal point if they are large compared to their radius of curvature; this problem can be mitigated by using mirrors small relative to their radius, approximating a parabolic profile that ensures rays reflect to a single focal point.