Android BLE DFU multiple devices update one after another...But some devices fails to update

Question

By using nordic Dfu library (updated) I have to update firmware of devices in one go,I have mac addresses of devices which required to update, let say i have 30 devices with their mac addresses and have to update one after another, By using nordic dfu library i got succeded to update 10 to 12 devices but rest of them got fails to update by throughing DFU Error message : DFU DEVICE DISCONNECTED when dfu library start to connect after start bootloader or sometime 1st round complete and in 2nd round it got fails.

I have tried some workaround as following:

1. After each failure i wait around 5 seconds and then try again to connect, these tries i did 6 times after that i skip that device and try to connect to another device with another mac address.
2. I did small scan just to refresh the device status.
3. On each disconnect call back i close the gatt. (gatt.close())

Now what i am missing or work around to update maximum devices , no matter if not update all 30 but atleast 25 to 27 devices should update. Sometimes i got 25 devices update but most of the time only 12 to 15 or 10 to 12 devices got update , rest of them fails.

Kindly help me, any help or workaround is appreciated.