Occasionally, pumps that are left in the resin and hardener containers become sticky and dispense components unevenly. If this happens, remove the pumps from the resin and hardener containers and soak the pumps overnight in white vinegar to remove any sticky resin or hardener. After soaking, wipe off residual vinegar, and let the pumps dry completely before using them again.
A set of resin and hardener pumps is fairly inexpensive. You'll have to decide whether it's worth the several hours you'll spend soaking the pumps, and ensuring proper disposal of the vinegar used for cleaning.