Maps Voice Navigation Not Always Working

I got a message from my mother today. She says.....

"Very frustrated with my iPhone map. I programmed an address and it went in great but it won't always talk to me. Don't know what I do. Sometimes she talked and sometimes she doesn't. Sometimes it's great, but I can't count on it. Any suggestions."

I don't use Maps in a car enough to answer her. Has anyone else encountered this and/or know of any solutions? Thanks, all.