If the files open in Nikon ViewNX 2 but not reliably in Adobe apps, the issue may be with file transfer or media integrity rather than the camera RAW format itself.

Good troubleshooting steps are:

• Try converting the NEF files with Adobe DNG Converter. If the DNG works, that helps isolate the problem.
• Test with a different memory card.
• Do a full format of the card in-camera before reuse.
• Try a different card reader.
• Try a different USB cable if you transfer directly from the camera.
• Test on a different computer.
• If possible, compare with files from another Nikon D3100 to see whether your system reads those normally.

A failing reader or cable can corrupt transfers even if the original file on the card is fine. That would also explain why Nikon software sometimes appears more tolerant than Adobe software.

So the most likely path is to verify the transfer chain first: card, file system, reader, cable, and computer. If the original files are intact, converting to DNG may also provide a workable short-term solution.