Sometimes when the simulation is long and you still only look at a limited set of waveforms the amount of data is so low that it is not flushed to disk, but instead kept in memory.
Sometimes it helps selecting and reload the results in the simulation window. (Simulation > Results > Select ...)
You have also the flushtime option in the transient setup to force spectre to save data more often, which I think can also be set by some environment variable to pick it up in the flow.