The decode button works only when the bodies are encoded/compressed with known formats. Fiddler Everywhere detects the format through the information received in the response (e.g., through headers like Content-Encoding). The body in the screenshots probably uses an unknown format or custom encoding (as it seems that part of the body is in readable form and another part is encoded).

If possible, send us a sample HTTP request reproducing the issue (that returns a response that can't be decoded) so that we can investigate the case further.