Google Maps is not like a normal sat nav. Sat navs have the full mapping stored in memory and only need GPS to operate fully.
Google Maps are not stored on your device so must download data, which it does when you first set the route. In theory it won't need to use any more data as long as you stick to the route but, if you detour, and it has to recalculate, it can only do it by downloading data again.
Could it be that your problem revolves around data download? If you haven't got a good phone signal, or data transmission is turned off, it cannot recalculate if you go off the original route.
For this reason I use Co-Pilot which, like a normal sat nav has the full map on my phone. When in Europe, where data download is very expensive, I can use Co-Pilot with data transmission turned off, just like a normal sat nav.