iOS Framework does not notify BLE event to my BLE central app

I am developing a app which implements BLE central manager. One of the feature of this app is to transfter file to a wearable device(Peripheral). The file size is around 25MB. All the files is not sent at a time. We chunks the file of 3000 bytes and after sending 1 chunk, device send ACK. then next chunk is sent. It works fine most of the time except for sudden case while iPhone LCD is off, file transfer is stopped. After investigating from BT air log, we found that device sends ACK but iOS FW does not deliver it to our app. During LCD off, app goes to background. It seems iOS FW does not deliver BLE event when app is in background. Our app supports UIBackgroundModes as bluetooth-central and bluetooth-peripheral. Can someone please let us know the solution of this problem?