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When there is NO flow through a boiler (like your second image for summer), the inlet and outlet temperatures will remain at the same value from the previous timestep where there WAS flow. The boiler does not store a volume of water, so there are no losses to surroundings like for a water heater. You can read more details about the hot water boiler model within EnergyPlus in the Engineering Reference.

When there IS flow through a boiler (like your first image for winter), you should have a temperature difference between inlet and outlet. If you are concerned that the temperature difference between inlet and outlet is smaller than you would expect, you have a few things to investigate:

1. Are there cooling loads from the fan coils during the winter week? If there is enough solar exposure and internal gains, this could be the case. This would cause a lower overall heating load for the hot water system.
2. Is the boiler flow rate (~35 kg/s or ~16 gpm) what you would expect? If not, then you should review the design water flow rate of the boiler.
3. Are the sizing inputs for the hot water loop defined correctly? You can access these by clicking on the horizontal dashed line in the OpenStudio Application diagram for the hot water system found in the HVAC Systems tab. You can read more about these inputs in another Unmet Hours post.

It's hard to tell without reviewing your model. In the future, you can also add a link to your post where others can access the input file from Google Drive, Dropbox, or a similar file sharing service.