Haveing trouble with my ar15 rail and iron sights?

I bought a UAG AR15 AR-15 M4 CARBINE RIFLE QUAD 4-WEAVER/PICATINNY RAIL and a set of Magpul MBUS Gen 1 sights but the front sight will not fit onto the quad rail but the rear one will fit on my upper receivers rail. What a not terribly expensive quad rail that will fit my slip up sights?
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