- The same grid is used also for layout, so you may want to have more options when routing tracks. But it depends on the pitch, track width and clearance in the area under BGA.
- Maybe you have too many errors, so Altium stops checking for the other errors? Go to your PCB: Tools -> Design Rule Check, try to increase "Stop when XXXX violations found" (I oftne set it to 1000), possibly if you are on the beginning of design and your board is unrouted you may want to got to Rules to Check -> Electrical -> Un-routed Net and disable it. Then press "Run Design Rule Check". Let me know if that helped.