BLE_GATTS_EVT_HVN_TX_COMPLETE is never called

Question

Hello, I have a bit of a weird case. I'm sending data over Bluetooth. In the begging, I wrote whole data handling in the main.c main loop. everything worked nicely, so I have decided to move all data handling to the new c file (MLX.c). Data is handling correctly, I have tested that using nrf_log, however sending it over Bluetooth I have an issue. it sends only 15 samples and stops while through nrf log I can see that data is still being handled. 
 Data sampling in MLX 
 case MLX_step:
 nrf_gpio_pin_clear(LED_1);
 sample = sample + 1;
 data_length = Data_length_setup(sample);
 sprintf(csample1,"s%d, \r",sample);
 BLT_data_send(csample1,data_length); // Data send over bluetooth

 for (int32_t a = 0; a<10 ;a++)
 { 
 int16_t test = MLX_read();

 if (test < 0)
 {test = test*(-1);}

 data_length = Data_length_setup(test);

 sprintf(mybuf,"%d, \r",test);
 BLT_data_send(mybuf,data_length); // Data send over bluetooth 
 nrf_delay_ms(10);
 }
 nrf_gpio_pin_set(LED_1);
 break; 
 
 BLT_data_send(); function is in the main.c 
 void BLT_data_send(char *data, int16_t data_l)
{ 
 uint32_t err_code = ble_nus_data_send(&m_nus, data,&data_l, m_conn_handle);
 //while (err_code == NRF_ERROR_RESOURCES);
} 
 
 I have figured out that I have issues with NRF_ERROR_RESOURCES, so I have added a new case in the "ble_evt_handler", however, I never seen "send" in the terminal. So I guess BLE_GATTS_EVT_HVN_TX_COMPLETE never occurs? 
 case BLE_GATTS_EVT_HVN_TX_COMPLETE:
 NRF_LOG_INFO("send");
 NRF_LOG_FLUSH(); 
 break; 
 
 I'm quite new with NRF and software and I can't really figure what's going on, especially then data handling is done in main.c file is all good. I assume some kind of Bluetooth buffer getting full or something. Any help would be appreciated, thanks! 
 I'm working on a custom-made board with the nrf52 module and Bluetooth is sent to the PCA10056 development kit. 
 Regards, Jonas G