A client may not be able to perform full AROM for a variety of reasons. Problems may include muscle weakness (agonist/synergist), soft tissue restriction (in the joint capsule or ligaments, or as a result of muscular contractures), or a bony blockage.
Observation during ROM
It is important to observe the client throughout the whole procedure. In addition to checking how far the client can move a body part, also pay attention to the quality of movement.
AROM only tells you whether the client can perform the movement or not! It does not provide information about what causes the lack of movement.
You may already know from the client interview that their movement is restricted.