Life is full of annoying things, and for most of us it is too short to bother with Greek accents (or at least, that's what we tell ourselves). So let me confess my latest act of utter despair, I had a look at the phrase πρός σε. Σύ is one of those words that has accented and unaccented forms. In its unaccented form (only available in Gen., Dat., Acc.) σύ throws its accent back on the previous word; it is an enclitic. According to Moulton II.180 the enclitic forms of the personal pronouns ἐγώ and σύ 'are not used after prepositions, except with πρός (generally).' This is confirmed (well, repeated at least) by BDF par. 279.
I searched my Bibleworks for the phrase in the NT (19 hits) and checked each of them against my trusted, printed NA27 (no differences).
Here is the result: πρός σε in the first two instances (Mat 14:28, 25:39), in all remaining 17 cases both words are accented.
Is there an explanation (or even a MS tradition!) or are we, in the words of the Dutch painter Karel Appel, 'just messing about'?