When you are capturing plans, and a query has the same query text as an already-saved plan, the existing plan is not replaced unless replace mode is enabled. If you have captured abstract plans for specific queries, and you are making physical changes to the database that affect optimizer choices, you need to replace existing plans for these changes to be saved.
Some actions that might require plan replacement are:
Adding or dropping indexes, or changing the keys or key ordering in indexes
Changing the partitioning on a table
Adding or removing buffer pools
Changing configuration parameters that affect query plans
For plans to be replaced, plan load mode should not be enabled in most cases. When plan association is active, any plan specifications are used as inputs to the optimizer. For example, if a full query plan includes the prefetch property and an I/O size of 2K, and you have created a 16K pool and want to replace the prefetch specification in the plan, do not enable plan load mode.
You may want to check query plans and replace some abstract plans as data distribution changes in tables, or after rebuilds on indexes, updating statistics, or changing the locking scheme.