There may be a few reasons for this. Have you read up on any methods to achieving full splits? Tom kurtz's book "Stretching Scientifically" is a must read!
Try tilting your hips forward as your stretch out. Also strengthen the main adductor and abductor muscle groups. (inside and outside thighs)
If your really keen. Get some advice from a dance instructor or gymnast. They help people do the splits regularly!