nRF51822 Tx power not able to set

Hello iam using softdevice 110 and SDK 10.0.0, Iam trying set tx power of nrf51822 using function sd_ble_tx_power_set(), i have tried 0dBm and 4dBm both are giving same RSSI and range, is it really configured or not how to confirm? The API is returning NRF_SUCCESS. Is there any pre-condition to set tx power? And i would also like to know when does GATT error 0x85 occurs, when i try to communicate from 20-30 meters this error occurs and bonding information is lost on central side(may be central is going out of range). Is there any way to avoid GATT error 0x85, because if central bonds again the peripheral is again storing bonding information in eeprom.