If all other things work fine on the phone, I would then check with the app creators to see if there is an update for it, or if there are things your phone doesn't have that the app may need to run properly.
Should that not be the problem, or if there are also problems with access for other things on the phone, then I would check with your carrier about your data speed/connection. Or you could try turning off 'data' on the phone and try accessing it while connected to WiFi instead.