The technical definition for unexplained infertility is, cases in which the standard infertility testing has not found a cause for the failure to get pregnant. Unexplained infertility is also referred to as idiopathic infertility. Another way to explain it is that the diagnostic testing we have available will only identify the major reasons why a couple may have a difficult time getting pregnant. They certainly cannot identify all the reasons, especially when infertility can be caused by other systems in the body, like being pre-diabetic or having high, unstable insulin levels. If the fallopian tubes are blocked or there is no sperm, these are obvious major obstacles to becoming pregnant and can be identified more easily.  
It is almost like limbo for a lot of couples. It now appears that 1-in-3 diagnoses receive the 'unexplained' diagnosis. According to Dr. Nobert Gleicher of the Centre for Human Reproduction USA, there are generally three main overlooked explanations for unexplained infertility:
1. Premature ovarian aging
Also known as diminished ovarian reserve, this condition is marked by follicle-stimulating hormones or anti-mullerian hormone levels being lower than they should be, in consideration of the patient’s age. For anyone suffering with low AMH.
2. Tubal disease or Endometriosis
According to Dr. Gleicher, not all hysterosalpingograms (a test for determining tubal efficiency) are conducted or read correctly.
3. Immune issues
Autoimmune conditions can occasionally result in a woman’s body rejecting pregnancies at an early point, meaning she may experience chemical pregnancies while still going on to have her period on time. 
As we noticed in our own clinical study, an antioxidant parameter known as Glutathione Peroxidase was pretty low in the majority of the men. Glutathione Peroxidase, made up of amino acids, has a protective and supportive role for sperm and egg cells. If Glutathione is low or depleted, then the cells do not get the protection they need - they will get damaged. This leads me to what is known as DNA fragmentation. If the DNA in a sperm cell is damaged, that cell cannot fertilise an egg. This test is not part of a standard fertility test in men.
Zinc deficiency, or any other nutrient deficiency can impair fertility. For example, zinc helps to create sperm cells and helps to regulate motility. Zinc is not tested for at a clinical level and, according to a recent British Health Survey, nine in 10 men are deficient. Scar tissue, high alcohol consumption, excess caffeine, low grade inflammation, high stress levels and being over/ under weight can also be part of the reason why someone hasn't conceived or is diagnosed as 'unexplained'.
Our fertility tests can only tell you so much. It is a snapshot of the current situation, and there are plenty of options to try and move the goalposts, some of which we have discussed before.
I obviously wish I had the panacea for the unexplained infertility diagnoses, and everyone has to make up their own mind on the best way to proceed for themselves. I certainly recommend making the necessary dietary and lifestyle changes, no matter how hard they might seem. Improving your dietary intake, reducing stress, balancing weight, and so on will have a supportive and corrective impact on the body. Exercise can also be crucial here.
Dr. Gleicher maintains there is always an answer. Finding that answer, sometimes simply involves delving deeper than the standard fertility evaluation would go. The diagnosis of unexplained infertility is not an indication that there is some mysterious underlying cause which cannot be discovered; it is an indication that we just haven't got to the bottom of the situation.
If we can identify the cause of an unexplained infertility diagnosis with more exhaustive tests, then that is a start and it might save you a lot of time and money in the long-term. We will then know what we are dealing with and can put a firm plan of action in place to improve that aspect.  
As always, I hope that this can help in some way.
Fertility Specialist