Visit Visa Rejection because of Security Reason.

Sir, I am from Pakistan, since 14 years am working here as a Civil Engineer in UAE, have applied two time for my younger brother visit visa but it was rejected. First time I have applied through a travel agent for long term(03 Months) in Sharjah but it was rejected and second time I have applied under my sponsorship through Emirates Airline for short term in Dubai But it has also been rejected. My Brother is young Fresh Graduate and before he did not come in UAE. I have tried to check from Emirates Airlines but they are saying that it is rejected from Immigration and we do not know. Please I have need your advice  that how I can find out the reason and clarify the situation.
        Awaiting for your advice and guideline please.