The 105mm Howitzer (and all artillery) has a fuse timer on the flares it can fire. For the 105 specifically the timer is 70 seconds from the moment you fire.
If the round's travel time is longer than 70 seconds then the flare dies mid-flight before reaching its target. If the travel time is 60-70 seconds then the flare ignites and only burns for a few seconds before dying, not giving off much light in the process.
This is only a real problem for the howitzer since it can have such long travel times for its rounds. Below is a list of all arty fuse times compared to maximum travel times.
105mm Howitizer
- 70 second fuse time
- Max of 77 seconds travel time
81mm M29 Mortar
- 100 second fuse time
- Max of 39 seconds travel time
60mm M2 Mortar
- 45 second fuse time
- Max of 28 seconds travel time