Attempting to retrieve a memory which is hard to access will have a greater impact on subsequent retrieval attempts than attempting to retrieve a memory which is easy to access.
Stated as conjecture in Robert A. Bjork and Bjork (1992), synthesizing a body of empirical research reported elsewhere (which I’ve not surveyed).
Bjork, R. A., & Bjork, E. L. (1992). A new theory of disuse and an old theory of stimulus fluctuation. In A. F. Healy, S. M. Kosslyn, & R. M. Shiffrin (Eds.), Essays in honor of William K. Estes, Vol. 1. From learning theory to connectionist theory; Vol. 2. From learning processes to cognitive processes (p. 35–67). Lawrence Erlbaum Associates, Inc.