If all the bulbs lite then you have one or more drawing less power. That happens if you get bulbs of different wattage and/or brand. Oshram a devision of Bosch is a good brand often found in OE installations. Many of the bulb sockets now have the wattage value next to the socket. If not match them side to side and throw out all brass based non rated bulbs.
With proper bulbs small amounts of corrosion can reduce the current flow and turn a 21w brake light into a 18w and the system will notice.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician