UIButtons don't align correctly upon first load of View Controller

UIButtons don't align correctly upon first load of View Controller

I have a ViewController/View in a TabBar Controller that has 8 UIButtons
in a 2x4 grid, sized 75x75, each with a 75x75 background image set on
them. Currently, I've hard coded the XY positions in the Size Inspector in
Interface Builder.
When I first load the View, by clicking on the tab bar icon, the bottom
two buttons get squished against the row above them - as per this picture:

Each button triggers a segue that launches another ViewController with a
WebView in it :
[self performSegueWithIdentifier:@"playVimeo" sender:sender];
In that WebView, there is a back button that triggers another segue to
return to the thumbnails page :
[self performSegueWithIdentifier:@"returnToThumbnails" sender:sender];
When I press the back button and return to the thumbnail view for a second
time, the buttons snap to the right place, unlike when they first load, as
per this picture:

Here is a screenshot of my Storyboard:

Does anyone have an idea what is going on here ? I am lost...
Thanks in advance!
Jesse