You said you connected all of the positive terminals first. I assume that this includes the + lead from the solar panel. If the negative lead from the sol ar panel is attached to the negative bus you now have a complete circuit. I assume that normally you disconnect all of the positive connections but the solar panel from the battery for the winter, thus isolating your battery monitor
so it is not seeing the power from the solar panel. When you connect all the + leads together at the battery it can now see the power from the solar panel. The solar panel does not require a battery in the circuit to produce power, just a current
path from the + to the - leads.