I need someone to help me. (Belgrade)

I'am trying to bring my girlfriend and his son from Senegal here.
I'am a Spanish citizen and they are Senegalese.
Last week she aplied visa form in Rabat Morocco ,for a business visit – invitation letter by a company in Serbia, but was rejected.
I'am engineer and my company is working on right now for Koridori.
I have temporary residence in Serbia, employment contract and I have rental housing contrac.
I think is better apply a private visit – invitation letter certifed by the relevant authority of the Republic of Serbia.
She's been twice in France whit Shengen Visa without problem.
I need someone to help me.