Hi Megan! There are a couple more things you can try to figure out what's going on.
1) The simplest idea is to try a new USB cable, sometimes it's just a finicky cable.
2) Assuming it's not the USB cable, you'll want to figure out whether the problem is with the Adalogger board or with the FED3 circuit board. To do this, remove the Adalogger board from one of the troublesome FED3s and see if you can flash the Adalogger while it's not installed in the FED3. If you can flash it this would suggest the issue is with the FED3 board. You can then confirm this by swapping in an Adalogger from a working FED3 onto a non-functioning one - if the problem still persists with the working Adalogger you'll know for sure it's something to do with the FED3 board. A few months ago I recorded a video
on how to do this!
Can you try those? I presume once you figure out what the issue is you can get reimbursed or replacements from OEPS. Post back if you figure it out! -Lex