first and foremost, your hands should cross over one another at the end of the movement, hence the word crossovers.
next you may be using to much shoulder movement putting stress on your arms and shoulders. i would recommend using a lighter weight and getting used to the exercise before putting to much stress on your shoulders and arms.
you may also try placing a flat bench between the cables and lie down doing crossovers (a very good alternative). and make sure at the end of the movement you crossover your hands.