This is likely due to the posts that aren’t displaying being assigned to the category that you have assigned to the Slider. You can correct this by doing the following:
Navigate to Appearance > Customize > Slider and check the category you have selected in the Post Categories field. If you don’t see this field then select “Default Slider” from the Slider dropdown, this will reveal some additional fields.
Any posts that have been assigned to the category that is currently selected will not display in the Blog. Even if the slider has been deactivated these posts will still be filtered out to prevent any unwanted posts suddenly showing up in a user’s blog should they choose to disable the slider at some point.
Once you know what category is being used for the slider all you need to do is go to the posts that aren’t displaying and unassign them from this category.