The most likely cause is third-party or ill-fitting cases obstructing the contact between the stylus and the tablet charging mechanism. Some cases may not be designed with stylus charging in mind, particularly if users are purchasing their own accessories.

Option B: A software update might fix OS-related issues, but would not typically affect physical charging.

Option C: If some devices had defects, the issue would likely be consistent, not isolated to some users.

Option D: Firmware issues could be a factor but are less likely than physical obstructions when users are using different accessories.

CompTIA A+ Core 1 Exam Objective Reference:

Objective 1.4: Given a scenario, configure settings and use cases for laptops and mobile devices.