Factors Limiting Flexibility

Flexibility or mobility, to use another term, is affected by both internal and external factors. It's important to be aware of both.

Internal Factors

The type of joint you want to stretch. Some joints just aren't meant to be flexible
The internal resistance within a joint
Bony structures which might limit movement
Previous injuries that have caused scar tissue to limit the elasticity of the muscle
The limited elasticity of tendons and ligaments. Ligaments don't stretch much and tendons shouldn't stretch at all
How elastic is your skin? 
The ability of a muscle to relax and contract within a range of motion
The temperature of the joint and associated tissues. Joints and tissues stretch more easily when the body temperature is one or two degrees higher than normal.

External Factors

The temperature of the place where you are stretching - it's easier when the atmosphere is warmer
The time of day in that most people are more flexible in the afternoon rather than in the morning after hours of inactivity (sleep)
If you are recovering from an injury it will be harder to stretch until the injured part is healed
Age - pre-adolescents are usually more flexible than older adolescents or adults
Gender - females (sorry guys!) are usually more flexible than males
Your ability to perform a particular exercise in that practice makes it easier
Your commitment to achieving your optimal flexibility
The restrictions of clothing and/or equipment used.

Some of the other factors that might impact on your flexibility include: Your bone structure which can contribute to the inflexibility of joints. Muscle mass, where the muscle is s heavily developed that it limits the ability for a complete range of motion to occur. Excess fatty tissue around a joint may also inhibit a good stretch. It is also easier to stretch when your body is well-hydrated. If your muscle tissue begins to "dry" for lack of water, it will be more difficult to stretch adequately.

Over Flexibility

Be careful. It is possible for the muscles of a joint to become too flexible. There is a tradeoff between flexibility and stability. The looser you get, the less support offered to the joints by their adjacent muscles. Excessive flexibility can be just as much a liability as not enough flexibility. Either one increases your risk of injury.

Once a muscle has reached its absolute maximum length, attempting to stretch the muscle further only serves to stretch the ligaments and put undue stress upon the tendons (things you DO NOT WANT TO STRETCH!) Ligaments will tear when stretched more than 6% of their normal length. Tendons are not even supposed to be able to lengthen. Even when stretched ligaments and tendons do not tear, loose joints and/or a decrease in the joint's stability can occur. This greatly increases your risk of injury.