Is your sprite continuing to slide when you're not pressing a key and the animation has stopped?
If so, that sounds to me like you have the player deceleration set quite low (the sliding part) and are perhaps triggering the walk animation using a key_down event where a platform_is_moving would work better (the animation stopping bit).
Unless I've interpreted your issue completely wrong. <img src="smileys/smiley1.gif" border="0" align="middle" />