My digital thermostat is not triggering the fan to blow when the a/c is turned on. I can get the fan to turn on by itself, and when in heat mode, but not when A/C is on. The unit is running out behind the house, and the outside fan is running. Please HELP!

What is the brand of your furnace? There are different solutions for different furnaces. One thought is that there is a problem in the thermostat itself. If you have the fan switch in the on position and you turn the thermostat to where is turns on the outdoor unit, does the fan in the furnace continue to run?
Here is one thing for you to try. Unplug your furnace from the electricity, go to the thermostat and move the heat-off-cool back several times very fast, then go plug the furnace back in and see if it made a difference. The reason for doing this is sometimes corrision puilds up on this little points and need to be cleaned. Post again if you need more help