Then what is the cause of absence of hard gamma in LENR according to your opinion?
The absence of hard gamma and further the absence of most of the energy one would expected based on the amount of transmutation is a good question. One must measure the amount of transmutation to see that. Most so called LENR experiments don't quantitate the transmutation. LENR detractors claim nuclear ash and high energy particles are a must for a nuclear reaction so they also don't measure the amount of transmutation on the basis no massive energy production then no reaction.
An electric arc though water produces nitrogen as a transmutation product. (See mass balance and stoichiometry in the ref in my profile). I note that so one can verify the absence of hard gamma and further the absence of most of the energy one would expected based on the amount of transmutation. The stoichiometry deduces from the data a specific reaction. I have referred to a generalization of that type of reaction as a Kidman type reaction. A Kidman type reaction occurs because the target and projectile atom's nuclei are re-arrange so that nuclei's charges are no longer point charges. That causes atom to atom bonding as magnet to magnet. That also causes screening that reduces the coulomb barrier.
In my opinion the absence of hard gamma and further the absence of most of the energy one would expected based on the amount of transmutation is because when these rearranged nuclei fuse, the expected energy emerges as an as yet undefined mass. In general mass production should follow the hierarchy of decay from a virtual Higgs particle to the zoo predicted by the standard model. But what happens here is likely more fundamental that the standard model. I speculate that if we had good candidates for what particles are produced by a Kidman type reaction we would also have a good candidates for dark energy and dark matter.