Reason #1: You don’t have their posture broken down enough. Their posture has to be broken keeping both hands on the back of the head, or one hand on the head and the other controlling his arm.
Reason #2: You are locking your leg over the foot, and not the ankle. If the triangle is locked over your foot, it is much weaker.
Reason#4: This is where a lot of people mess up. You have to cut a perpendicular angle by grabbing an under-hook on the leg or underneath their free arm. If the angle is not cut, they will have more ability to posture or pressure into you.
Reason #5: Pressure isn’t being applied correctly to finish the choke.
Below is a link to UFC fighter, Ryan Hall, showing the proper way to apply pressure for the choke:
The Arm Bar Blog 