BLE Nano keeps disconnecting after each transaction


I have built a connection between BLE Nano and the android phone.The BLE will sample some data (about 200) and process them and send 8 bytes to the phone. Previously when I have the same software and firmware code, it was fine most of the time except if the sample size was bigger it disconnected. Now I have a new hardware connect to the BLE. It worked OK for a while and then started to disconnect on every transaction. The disconnectionCallback code is 536887080. Does anyone know what causes this? Thanks.