It’s likely the antivirus products are detecting the usage of MPress itself as a sign the file is malicious.
Executable packers create files which are usually significantly different from “normal” programs in a few ways:
- the headers are minimised, the MPress site claims to remove standard features like debug directories and relocations.
- executable sections have high entropy (due to compression) and usually minimal space between them (a normal PE has a lot of null bytes).
- code sections often have read/write/execute permissions, or code is created in memory with write permissions and then executed. This allows code to be decompressed in place but is unusual (outside special cases like JIT compilers).
Other reasons antivirus programs may detect these files include:
- they are often used by malware, and rarely used enough for vendors to worry about false positives (users can add exceptions).
- there are few compelling reasons to use packers nowadays, as storage sizes and download speeds exceed requirements for most programs.
As for the reasons files can’t be scanned, this is vendor specific. Perhaps something about the ZIP file is unusual, and the engines are refusing to scan to avoid problems. If you supply hashes of your files it might be possible to compare them further.