Due to our recent IUI failure I've been hearing this question alot the past few weeks. I thought it might be a good opportunity to discuss reasons why some IUI's fail. From the articles I've read, success rates vary between 5% to 25%.
AGE:
Like most fertility treatments, increasing age of a woman (35 and older) decreases success rates. Unfortunately egg quality tends to decline with age. At age 18, a woman has a 25% chance of conceiving, however by age 40 her chances decline to about 4 to 6%. A woman also has less eggs or ovarian reserve as she ages since many have already been used with previous cycles (number of eggs left = ovarian reserve).
EGG QUALITY:
Just because a woman is ovulating doesn't mean that she has quality eggs. Low quality eggs have internal defects such as chromosomal problems or problems with energy production (eggs need to be able to divide after fertilization). A woman is born with about 1 million eggs that like dormant in the ovaries until they are "activated" by hormones at the time of ovulation. Unfortunately as far as I know of there is nothing that can "boost" egg quality. Often times, couples with "bad eggs" need to rely on donor eggs. From my understanding the only way to truly test egg quality is with egg retrieval prior to completing an IVF cycle. The bad news is that this is very costly financially and invasive (requires a surgical procedure). Also, after the eggs are retrieved there is no accurate test to assess quality. Often times it is subjective and based on someone looking at them under a microscope. Since egg morphology has not been adequately studied, many women go through several cycles of failed IUIs or IVF cycles without even knowing she has poor egg quality.
Last, eggs that are released may be immature and therefore incapable of being fertilized. During a normal cycle, certain hormones prepare the egg to have the capability of becoming fertilized.
BLOCKED FALLOPIAN TUBES:
Blocked fallopian tubes can occur from many factors such as STDs, a uterine infection caused by abortion or miscarriage, a ruptured appendix (causes scar tissue), any previous abdominal surgery (causes scar tissue), ectopic pregnancy or endometriosis. If the tube is blocked, there is a physical barrier preventing the egg and sperm from coming into contact. Luckily an HSG test, although invasive and painful, can detect this. Many times the solution they inject into the uterus during the procedure can unblock the tube. If that fails, sometimes laparoscopic surgery is required to remove scar tissue.
POOR TIMING
Timing is critical. After the egg is released from an ovary it only has about 12 hours to become fertilized. If fertilization fails the egg disintegrates. It's amazing anyone becomes pregnant with such a short time frame allowed for fertilization. The good news is that sperm can survive for a few days in the woman's uterus. If intercourse occurs a few days before fertilization, the sperm can already be there to wait for the egg. Also, with intercourse sperm can travel up the wrong fallopian tube or can become trapped. Sperm can also swim poorly for various reasons, not allowing them enough time to reach the egg.
The IUI must be done in roughly that 12 hour window. If the IUI is done too early or worse too late (after ovulation has occurred), the chance of the sperm contacting the egg is slim. IUI has the advantage of helping the sperm get to it's target quicker, however if the egg has not yet matured or has already died the procedure will fail. To ensure this doesn't happen, it is essential that the woman has several bloodtests to check hormone levels that indicate she is ovulating. Ultrasounds are also essential to determine the exact time of ovulation. Nowadays many doctors help mother nature by giving an injection of HCG - a hormone that triggers ovulation. This takes much of the guess work out of the equation....but remember if the shot is given too early the egg will be immature. If the shot is given too late, natural ovulation may have already occurred.
NOT OVULATING
If a woman isn't ovulating there is no egg to be fertilized. No doctor in the right mind will complete an IUI unless ovulation has been triggered with hormones.
POOR SPERM QUALITY
This is our problem. Even though there may be ample sperm, if they are malformed they may not have the physical capability of penetrating the egg to fertilize it. This is usually caused by deformities in the head of the sperm. It's like having a million spoons when all you need is a knife. The spoon just won't do the trick. Many studies have shown poor success rates for IUIs with poor sperm quality. Sadly there is no known medical treatment to improve sperm quality.
WHEN IS AN IUI SUCCESSFUL
LOW SPERM COUNT OR MOTILITY:
Sperm can also swim slowly, may not be able to swim at all, swim in circles going no where, or may have 2 tails causing them to swim poorly. If they can't reach the egg in time, it'll be too late. Luckily for this particular problem the IUI does the work for the sperm. Since the sperm are injected directly into the uterus they need not swim far. Also, if the sperm are normal but there just isn't an abundance of them, IUIs are more successful as it gets the few sperm closer to the egg.
I am completely heartbroken! All I want to do is cry 
