Stuart makes a good observation. Even with a proper setup there are holes in the network of digital repeaters that leave your APRS position hanging in the wind. Northcentral Kansas is definitely a prime example. Once I left the Manhattan area last week on a chase, the digipeaters became scare leaving my position to be shown just outside of town.
Another issue with showing up on FindU is not only digipeater coverage, but coverage that can lead you to an internet gateway. Not all digipeaters are connected to the internet, and unless your packet bounces around to be caught by one that is, you will not show up on FindU.
Definitely post questions you may have with regard to specifics, there are a number of APRS users here.