Here is the progression of this ailment:
1. Sometimes you have to push the defrost button a second time to get it to work.
2. You have to push the d@*n buttom repeatedly, but you finally get it to work.
3. You have to push real hard, and sort-of to the left, and you can get it to work - sometimes.
4. No matter what you do, the defroster refuses to work.
There is no need to pay a bunch of money for an expensive diagnosis. IMO, Nissan cheaped out and used junk components and poor assembly techniques to manufacture a sub-par A/C Controller assembly. Because of a cheap push button switch that is soldered into a circuit board assembly, a new controller will be required. This new controller is the better part of $300, and figure on a couple hours labor. This means you better have an extra $500 laying around, because of a cheap component that probably cost less than a buck.
The controller on my 2008 is, I suspect, an older model. The catalog part number shows an older number that has been superseded by a newer number. We can only hope that the newer number has better components. A very visible difference between my old one and my "new" one (more on that later), is the new one has some matching "orange colored" graphics to encourage you to keep the "Intake Air Selector" on "fresh" when using the defroster.
I just finished this repair today. First, the problem is a switch that is soldered into a circuit board on the back of the A/C Controller Assembly. There is no practical way to repair this switch nor swap out any sub-assembly. The entire controller (which consists of the 3 rotary knobs and the 2 push-button switches) must be replaced. The switch fails in the same way a single button will quit working on a TV remote control. I purchased a used controller from a junkyard.
To swap out the controller involves taking off the top "Instrument Finisher E", the "Cluster Lid C", the "Audio Unit", the "Instrument Finisher D", then removing the "Intake Door Knob", followed by the "Controller Finisher", and finally the "Controller".
The hardest thing to figure out is the 3 control cables, and how to release them. Each cable housing goes into a "socket affair" that has 2 plastic spring-loaded "grippers". These have to be spread apart to allow the cable housings to be extracted. I had success using 2 different types of snap-ring pliers. They both expand a snap-ring when the grips are squeezed, and I needed both straight and right-angle pliers. The cable housings must be inserted completely on re-assembly, or the grippers will not do their job and the cable housing will slide back and forth. It may not be abundantly clear, but the ends of the plastic cable housings are "molded" and will only properly seat if the housings are rotated to 1 of 2 positions (there are flats on 2 sides that forms a bump that is gripped).
The other thing that gave me trouble was the big black electrical connector on the controller. The tab that must be pushed in to release the connector is on top, and almost impossible to depress with fat, stubby fingers. Persistence will get you through.
2 final thoughts: make sure you disconnect the battery cables (negative first please) or you will have an airbag error code when you are done (you have to disconnect the airbag warning light harness). Clearing this code is a PITA and involves considerable research to find the procedure. The second thought is, this procedure should not be performed around children, mothers, wives, and especially "church friends". Failure to heed this warning could result in the diminished perception of your character.