If I understand your question correctly, you will need to wait till you learn about chemical equilibrium. Basically, reactions almost never go to completion, rather, they reach an equilibrium, in which both reactants and products are present. Sometimes reaction is shifted so far to the products side we can assume it went to completion, but it is never exactly true.
Dissociation is just a reaction, sometimes it goes to completion, sometimes it doesn't. Why? It depends on many factors, but the driving force is usually the stability of the non-dissociated substance and dissociated ions - when the undissociated substance is more stable, it will prefer to stay undissociated, when the dissociations products are more stable, substance will tend to dissociate.