Turbine steam consumption and output doesn't seem right

I have one boiler, outputting 48/min high pressure steam. This feeds two high pressure turbines, consuming 24/min high pressure steam. These then output 24/min low pressure steam, which feeds two low pressure turbines. Each turbine (high and low) outputs 1MW of shaft power, which feeds two power generators per turbine, each consuming 500KW of power.
The high pressure turbine generators have the highest priority, and I have enough electricity consumption that they are running at max capacity (as verified by the output % on the generators).
So far this all seems balanced.
However, the low pressure turbines are barely turning, even after I routed all the low pressure steam to a single turbine. It seems that the high pressure turbines are consuming the right amount of steam (the boiler is working constantly) but only outputting a tiny amount of low pressure steam - hardly enough to start the low pressure turbine spinning.
What's going on?