Two things to check.
Is there a Route Keepout defined in that area which is causing the separation (turn all Route Keepout Layers)
Is there a shape void defined that is causing the separation. (Shape > Manual Void.... > Delete, select the shape and window over the separation to delete the void if it exists) Also running Delete Islands (Shape > Delete Islands) would create these types of shape voids if at one point you had two traces that ran thru the area to form an island in the shape.
Hope this help,
Plexus Engineering Solutions