This page of the Deathloop game guide contains a solution to a potential problem - why visionaries don't appear in their default locations. You may often encounter this problem during the game and it can make it difficult to eliminate the visionaries to progress the main story.
This page contains minor spoilers about how the visionary hunt is organised and what changes can occur in the leads. However, we tried to avoid major story spoilers.
Each of the visionaries (except for Julianna) resides by default in one of the 4 Blackreef districts. However, once you reach the area "associated" with the given antagonist, you may not find them there at all.
By far the most common reason why a visionary doesn't show up at a particular location is because it's the wrong time of day. Selecting the wrong time of day may result in the boss not appearing on the map or their location may become temporarily unavailable.
Most of the time, you can confirm the correct time of day in your character's journal. Lead descriptions mention the time of day when they become available and this also applies to visionary leads. Always double check this information.
You can also check when the visionaries are available in the mission preparation screen. In the example from the screenshot above, the information shows that 2 visionaries will appear in the Updaam district in the evening. If the visionary field doesn't appear at all, it means that you won't encounter any of the main antagonists in a given district at a given time of day.
There is another reason why a visionary may not appear in their default location - Colt's actions may have changed the visionary's day plan. For example, if you perform certain activities or sabotages in the morning, it may lead to the visionary appearing in another district in the afternoon or evening.
Paradoxically, the situation described above is not a hindrance, but a blessing. To complete the campaign, you must intentionally cause 2 or more visionaries to appear in the same area, i.e. in the same location and at the same time of day. This is because, in order to beat the game, you have to kill 8 visionaries on the same day (within the same Loop). Killing only 1 visionary in each location wouldn't give you enough time before the end of the day. As with the main issue, keep an eye out for any changes in the lead tab to make it easier to determine when and how to make multiple visionaries appear in the same location.