I am sending 6 packets of 20B each data using ble_nus_string_send() function at every 120ms using app timer. but the function returns error BLE_ERROR_NO_TX_PACKETS. So unable to find the exact issue that is causing this issue. As far as the calculations 1kB should not be an issue.
I am also confused that why the counter value always return as 1 of p_ble_evt->evt.common_evt.params.tx_complete.count.
these are the connection interval values I have used
#define MIN_CONN_INTERVAL MSEC_TO_UNITS(7.5, UNIT_1_25_MS)
#define MAX_CONN_INTERVAL MSEC_TO_UNITS(15, UNIT_1_25_MS)