Missing Edges in Section View

[smseals]

Good afternoon,

I am new to Solidworks and am using 2015 SP4.0.
I have created a section view, but seemingly random edges are missing.
This Hide/Show Edges command highlights all the missing edges, but wont allow me to deselect them.
Other edges can be selected & deselected as you would expect.
Changing the view type to a slice section shows all edges, changing back to a partial section hides all offending items again.

Can anyone shed any light on this / offer a solution ?

Thanks in advance,

Richard.