Sprint approved the HTC One M8 Lollipop update a few weeks ago, and now you can install the full system image manually on the off chance something went wrong with the OTA. You'll need a computer with the proper drivers set up, but it's better than not getting the update at all.
The drivers can be obtained by installing HTC Sync on the computer if the device is not recognized when you plug it in. After your phone shows up, launch the RUU and follow the steps in the install wizard. If you need to do the RUU install, remember it will completely clear files on the device. Make a backup and grab the 1.4GB RUU for your device below—standard or Harmon Kardon.