Sometimes theres a server delay and take a time to show achievements... 
In other hand, do you check the status of this achievement before? 

I just upradeda beacon to lvl 2and ten frm my smalle leaguei captured it for league achoevemet credit....the report only said i captured the beacon...it droped from level 2 to level 1 but no credit was given.....can you explan why so next timw we ddont waste resources

Can you send a screen shot of this happening? We can have developers look into it and they can check what is going on. As Chascon said, it is sometimes a delay showing the achievement. If it was downgraded by someone else the same time you hit it though it can show it as lvl 1 on your reports but another league may have got the downgrade. This has happened to me before. Disappointing but its all in the fun of the game :)