Yes, I've run into that before, but it doesn't necessarily mean the YIKONG YK4101 RC Car Servo (17G, Part No. 13119) is faulty. In many cases, the differential lock gears simply aren't aligned, so the locking mechanism can't engage immediately. Try rotating the wheels by hand or slowly rolling the vehicle forward or backward a short distance, then activate the differential lock again. It will often engage normally.
If it still won't lock after several attempts, check whether the YIKONG YK4101 RC Car Servo (17G, Part No. 13119) is operating correctly, whether the shift linkage moves freely, and whether it has been installed properly. It's also worth inspecting the differential lock assembly for worn parts, debris, or insufficient lubrication. In most cases, a quick inspection and adjustment will restore normal differential lock operation.