If the file does not end with .axx, one of two things apply:
1) The file is not in fact encrypted with AxCrypt.
2) You, or someone else, renamed them.
If you have tried to rename them, and they still are not possible to decrypt with the message “not an AxCrypt file”, then… situation 1) appear to apply. The files are not encrypted with AxCrypt.
You can also contact support and send screen shots of the errors and possibly a sample file.