Most likely there is one of several things going on here. 1. Different weapons have different levels of ability to track characters. For instance, a legendary rocket launcher with tracking will not be as effective at following a player as the Exotic Rocket Launcher Truth which "aggressively" tracks targets. 2. Multiple targets moving in different directions will decrease the tracking effectiveness of all weapons and "sticky" grenades. 3. Sticky grenades have a few weaknesses. Point #2 about multiple targets can cause the grenade to completely miss it's target as it attempts to resolve it's attraction to both enemies. Also, "sticky" grenades can be shot out of the are or stick to non player items (such as throwing knives). This is extremely hard to do on purpose but can happen. When this happens it can appear as if the grenade stuck to the player when in fact it was shot in transit very close to the player's body. Any combination of these issues happening in the timeframe of a few matches can make it seem as if the system is not working correctly.