Menu items not showing up

It does that because the amount of items in the grandchild menu.
There are a couple options here.
1. Restructure the menu so you have no more than 6-8 items.
2. Change some css styling so the list is vertically shorter.

I see you use the Jetpack plugin, you can go into the Jetpack options and activate the custom css module then enter the following code.

This will change the spacing between the text and the blue background border as well as lengthen the links so they don’t wrap 2 lines.

.main-navigation ul ul a {
padding: 4px 14px;
width: 11em;
}

This code will remove the boxy borders that surround the drop down menus, if you so choose.

.main-navigation ul ul {
box-shadow: none;
-moz-box-shadow: none;
-webkit-box-shadow: none;
}