iPhones typically go into sleep mode between 1-5 minutes. If you are running Fliq or Fliq Notes on the phone when it goes to sleep there is a possibility that the phone will remain available in Fliq for Mac or Fliq for Windows. The list updates depending on the speed of your network connection. In some cases you can select and send to the device even though it is not truly available, so, the transmission will fail. You should receive a warning message that the device is not available. Reopen Fliq on the device and resend if there are any problems. Remember to keep Fliq or Fliq Notes active on your phone if you are expecting to receive data.