I just finished installing an EchoUat in my RV-8. Your Garmin mode C has to be on and working 100% in order for the EchoUAT to pick up your pressure altitude and squawk code. You should see ALT and a blinking R on the front panel of the transponder.
The EchoUAT app will tell you what data components are currently being stored by the transceiver. You should be able to see Longitude, Latitude and GPS Altitude. If you don't see these data points, it means the transceiver is not receiving data from your WAAS GPS. If you don't see pressure altitude and squawk code data points, then the transceiver is not receiving data from your Garmin 327. When I initially installed mine the transceiver was not receiving any data from my Garmin 327 and I had to change the transponder threshold parameter to make it work.
Given the above, what data elements are you seeing on the app?
First Flight 11/13/2011
TMX0360, Pmags, CS