Overflow box Problems

Hello
I just put the sand water and rock in my tank, and i put the overflow box in.
The water goes over the overflow box, when i suck on the tube to start the siphone it wont go, anyone have any ideas why?
I tested it on fresh water a few weeks back and it worked!

If you need pics i can get some

 

Are both end of the U tube covered in water?

Did the U tube get cracked and you don't know it?

Are you inserting the air line so the tip is at the top portion of the overflow tube were it just begins to bend and make the "U"?

Are you pulling the air out fast enough to draw the water from the skimmer box to the overflow box in a rush?

 

I got the overflow working, but after a few mins air is getting into the U bend and obviously slowing the overflow!
any ideas as to why?

 

Not enough flow through your tank from the return is the most likely cause.
An unlevel skimmer box is another culprit.

 

The return is to quick if anything, so an unlevel overflow box can cause this? if so that will be the cause as it doesnt sit square properly. Any ideas how to make it sit squarely over the edge?

 

When you start the siphon, do you get all the air out?

 

realised there is a minature hole in a corner on the U bend, will seal it tomorrow.
Thanks for the help guys