BLE CSC Service not always detected - proper setup?

Question

I'm building a bike cadence sensor on an Arduino that connects via Bluetooth LE to a bike computer. It is reliably detected by a Bryton bike computer but not by an iOS smartphone app from Wahoo. This suggests to me that maybe the service is not properly set up. Does anything stand out in the code below or is there a good example how to do it?

#define     CSC_FEATURES                            CSC_FEATURE_CRANK_REV_DATA

unsigned char   bleBuffer[1 + 2 + 2];
unsigned char   slBuffer[1];
unsigned char   fBuffer[2];

BLEService      CSCService("1816");
BLECharacteristic CSCMeasurement("2A5B", BLERead | BLENotify, sizeof(bleBuffer));
BLECharacteristic CSCFeature("2A5C", BLERead, sizeof(fBuffer));
BLECharacteristic CSCSensorLocation("2A5D", BLERead, sizeof(slBuffer));

byte            flags = CSC_MEASUREMENT_CRANK_REV_PRESENT;
unsigned short  features = CSC_FEATURE_CRANK_REV_DATA;
byte            sensorlocation = SENSOR_LOCATION_RIGHT_CRANK;


BLE.setDeviceName(BLE_DEVICE_NAME);
BLE.setLocalName(BLE_LOCAL_NAME);
BLE.setAdvertisedService(CSCService);
CSCService.addCharacteristic(CSCFeature);
CSCService.addCharacteristic(CSCMeasurement);
CSCService.addCharacteristic(CSCSensorLocation);
BLE.addService(CSCService);
update(millis()); /* initialise buffers */
BLE.advertise();