abcm2ps normally makes automatic decisions as to where to place any given note, and usually that's fine.
But you can over-ride this for bits of the tune as you've done.
But easier still, you can force a voice to stay on one clef throughout, whatever the notes, by specifying the clef in the header. In your case for a single voice, assuming the key of e.g. Gmajor do this:-
K:G clef=treble (or whatever)
Refer to the abcplus_En manual here http://abcplus.sourceforge.net/#ABCGuide
for a fuller explanation. Scroll down the page a bit and download the PDF file.