Breaker has line voltage on the line side but voltage on the load side is low and variable
Occasionally a new circuit breaker never placed in service will appear to be open when closed, evidenced by no or low voltage on the load side with the line side energized.
Every set of contacts has a minimum voltage and current requirement to normalize and make them fully operational. This is why we have gold-plated low-level aux. contacts available for breakers--to permit operation at very low levels of current and voltage.
The circuit breaker main contacts are subject to the same phenomenon, and since they are designed for higher current and voltage levels, they don't function well at the extremely low values. While voltage on the supply side may be nominal, with no load connected the current flow is zero and well below the minimum required for proper operation.
This is not a problem for the breaker in service. If you place the breaker in service with at least 20% of the breaker's rated current, the resisitve film will be burned off and resistance will become negligible full voltage will be passed on to the load side. After this, the contacts will also conduct the lower values of current.