SAR 4800 won't close

[gadovid]

I just ordered a scope mount for my SAR 4800 FN copy. I figured out how to get it open and slide the old dust cover off, and put the new one on in it's place. Now, it won' t snap shut!HELP!!!!,

 

[IMDB]

Make sure the scope mount is all the way forward, Is the scope mount even with the back of the upper receiver?

 

[gadovid]

Yes, it appears to be even. I even gently tapped it with a rubber mallet to be sure.

 

[zombie_apocolypse]

pict