The most likely cause is that you didn't add all of the required configuration for New Relic as listed here: https://docs.newrelic.com/docs/mobile-apps/ios-installation-and-configuration#configuration
Add to your build phases:
CoreTelephony.framework
SystemConfiguration.framework
libz.dylib
In your [app_name]-Prefix.pch project file (generally found in the "Supporting Files" folder) include the New Relic header:
#import <NewRelicAgent/NewRelic.h>
In your AppDelegate.m file add this call as the first line of application:didFinishLaunchingWithOptions:
[NewRelicAgent startWithApplicationToken:@"<your app token>"];
Clean and build your build folder, by holding down 'option' and going to the 'product' drop-down in Xcode. (See the screenshot here)
If it's still not working, you'll probably need to get in touch with New Relic at support.newrelic.com