Do you click on your pikemen to attack, or just leave the enemy to engage. I've found with most units, if you leave them on hold formation just let the enemy engage, they tend to get pushed back easier and bunch up/distort their formation.

Giving them a simple attack order ususally rectifies this (always happens with hoplites and i've used it with phalanx) They tend to push back after that and the odds are a bit more even.