A full agonist is an agonist that is able to elicit the maximal effect by binding to all or some of its receptors. Partial agonists are unable to elicit the maximum response, even with every receptor activated.
When combining a partial agonist and a full agonist, the maximal effect may still not be able to be obtained. This is due to the partial agonist competing with the full agonist, suppressing its activity.