### Short answer
Use `-1` instead of `--portrait`.
### Slightly longer answer
By default `a2ps` tries to put 2 pages on each sheet, this works best side-by-side and landscape, even if you rotate the page `a2ps` still tries to insert 2 pages. The answer is to give `a2ps` the `-1` switch which instructs it to only have 1 page per sheet (`3-9` are also valid number of pages).