iPads stuck in DFU mode and not powering on

I work for a small business that fixes iPads for a refurbishing company. There is a handful of them (iPad 2s and 4s) that have been tossed aside as being not able to turn on and multiple different parts have been used in them such as new LCDs, new digitizers, new batteries, new charge ports, etc; and still do not power up. However, when plugging into the computer the PC recognizes these iPads and they are stuck in DFU mode and can't get out. The screen normally does not light up unless being prepared to restore , the apple logo will appear with a bar underneath it, but not always.

iTunes is never able to restore these iPads as they keep getting stuck on "Preparing iPad for restore" or "Waiting for iPad", and when I try to use ipsw files I get an error 3194 or 4014. I already looked at my hosts file and nothing unusual is there. I tried 2 different computers (both Windows) and new cords as well.

Has anyone encountered something like this before? Could this indicate the iPads were previously jailbroken? Is there a way to "flash" software onto the iPad, or force the software back onto the iPad not through iTunes? Sorry if these questions don't make sense- I don't know much about jailbreaking whatsoever.

Any help would be greatly appreciated, thank you