Gap in Model (Blender)

As you can see below, there is a little ‘gap’ in my model which is causing annoyance.
The vertexes are joined and I really don’t know how I can fix this, any help will be appreciated.
(Sorry about the size of them.)

The left ones are joined, but the right two aren’t.

How would I go about fixing it then?

I select the verts and hit ‘J’ but the neck still had no change in its slit.

Select the verticies in edit mode and press alt+m to merge them