At some point the job responsibility you once found very motivating starts to be less appealing.
In most organizations, job descriptions are ironclad walls of individual authority and jurisdiction, untouchable by anyone but the jobholder. So you're stuck with a task or project that no longer engages your best work and others are similarly trapped.
It doesn't have to have this way.
In almost every organization, some projects assigned to a particular individual or department could just as easily be done by others. Pull these projects out of permanent individual ownership and instead place them in a pool. Annually let people select a pool project (either individually or as part of a temporary team) that gets them excited, one for which they have new ideas and a lot of passion and energy.
By placing some work up for grabs each year, you ensure individuals always get some variety in their jobs, that projects are less likely to get stale, and that territorial walls around certain activities don't get built so quickly.