Development of doctrine is to be distinguished from new revelation. The former unquestionably takes place, but there will be no new public revelation that we must consider as an addition to that which we already have in Scripture and Sacred Tradition.
“The Christian economy, therefore, since it is the new and definitive Covenant, will never pass away; and no new public revelation is to be expected before the glorious manifestation of our Lord Jesus Christ.” Yet even if Revelation is already complete, it has not been made completely explicit; it remains for Christian faith gradually to grasp its full significance over the course of the centuries. [CCC §66]
The Church distinguishes between public revelation (as in Scripture and Tradition) and private revelations made to individuals, which do not under any circumstance add to the Deposit of Faith. So for instance the apparition of Our Lady of Fatima is a private revelation; it is “approved” by the Church, but the Church does not consider anything said by the Blessed Virgin there to constitute a supplement to be included in Scripture or Tradition. See for example CCC §67, at the link above.
On the other hand, the Church’s understanding of revelation is not complete. Over the course of time she understands the Faith more clearly. This is rightly and necessarily distinguished from revelation, which it is not; rather, it is the fruit of the Church’s contemplation and increasing comprehension of what God has revealed.