Search API View, search relation fields
There's 2 areas you need to check this is included.  It's common to have applied to the search and not realise it needs to applied in views also. 

I take it so far you have made the fields in 

configuration > search api > Operations 'EDIT'   

"Fields" tab > 'Add Related Fields ' at the bottom. 

Now you also need to add the Fields in 

structure > views > your view 

Filter Criteria >  

On the right hand side look for 'Searched Fields'  - make sure all the fields your searching are in here.  

I hope that solves it for you .