The fill is dependant on clearance settings - which can be set for the zone (in the zone dialogue), track (in the File->Board setup…) or an individual pad (press ‘e’ whilst hovering over the pad).
However, there must be a contiguous route for the fill to reach an area. You may have voids left in your layout if there is no way for the fill to reach that space. Try moving one of the vias that are just above the lower circled area - you will probably be able to get the area to fill. Alternatively, if you have GND plane on both sides of your boards, a few judiciously placed vias will stitch top and bottom planes together. What you don’t want is isolated islands of copper not tied to a net.
John_Pateman already gave some information, but there are so many details that generic explanations may feel abstract. In any specific case there may be several clearances and the minimum width which may affect filling, and sometimes even an experienced user may get confused with a problem, especially near pads.
If you give an example board file (reduced to minimum to show some example cases) we might be able to give detailed explanations which would help you to understand the system.