Question: Midichlorian's are chemical compounds found within every living cell that forms the basis between the connection life and the Force. Without midichlorians, life would be impossible. The amount of midichlorians in a life form represents its ability to understand, comprehend, and manipulate the Force.

My name is Darth Incognitus, and I have decided to absorb more of these cells into my own body to make my force sensitive abilities stronger. To accomplish this, I have kidnapped numerous force sensitive users who match my blood type. When their midichlorian count has been measured, I will perform a blood transfusion from them to myself in order to absorb these cells. After I have attained enough of these cells, I will become the most powerful force user in the world and create my Sith empire. Then, I will use this method to strengthen my apprentices and students.

However, after a number of tries, I have noticed that my powers have not increased. No matter how many Jedi I murder and drain of blood, my abilities remain average. Why would this be the case?

Answer: 
Because the blood level midichlorian equilibrium point is determined by your body's biochemistry. You can't alter this by adding more midichlorians. All that does is cause a temporary imbalance (way too short for your body to make use of it) that your body quickly fixes. Excess midichlorian gets filtered out by the kidneys and ends up down the drain. Literally.
**Because they are genetically foreign.**

They immediately start to reject the body, and the body rejects them. Welcome to a force fight at the cellular level.

Immuno-suppressants allow the transfusion to succeed without ripping your body apart, but the downside is that they significantly hamper even your own ability to interact with the force, injecting extra barely raises you back to functioning.

**Cloning ;)**

Okay, genetically identical. Except that the Midichlorian's are already at saturation in your body, adding more of the same simply causes them to die off.

You might, through an ingenuous system of continuous transfusions keep yourself in a state of over-saturation. This would amp up the basic capacity for force manipulation (you will still need practice as with any fine motor control), but there are (semi-)permanent side-effects. The Midichlorian's learn that massive spikes occur, and reduce their overall numbers so as to survive long-term. You might recover to normal levels, it might be a permanent reduction.

**Divergence**

As the force wielder learns, the Midichlorians alter themselves physically. Even if they were cloned and successfully duplicated, reintroduction back into the body causes the equivalent of a theological argument between religious sects. At best nothing changes, at worst valuable skills (learning) are lost as the two sides duke it out, or the host dies.

In short you are extremely lucky to be alive.