I was reading Wikipedia article on Bohr-Kramers-Slater (BKS) theory, https://en.wikipedia.org/wiki/BKS_theory. I encountered two interesting points and need your help to understand the reasons behind them.
Einstein was awarded Nobel Prize in 1921  primarily for his work on photoelectric effect which used photon model to explain the phenomenon.
BKS theory was put forward around 1924.
The Wikipedia article says the following.
Nevertheless, Bohr and Kramers had two objections to Slater's proposal:
1: The assumption that photons exist. Even though Einstein's photon hypothesis could explain in a simple way the photoelectric effect, as well as conservation of energy in processes of de-excitation of an atom followed by excitation of a neighboring one, Bohr had always been reluctant to accept the reality of photons, his main argument being the problem of reconciling the existence of photons with the phenomenon of interference;
Niels Bohr was one the leading physicist at that time; around 1924. I'm sure there would be others prominent physicists like him who didn't really believe in photons. I understand there are always some persons who tend to oppose or doubt even well established theories or models. In this case, the mention of Bohr really fascinated me. I know that Einstein and Bohr had totally different interpretation of quantum mechanics.
The article also says the following.
However, physically the most provocative element of the theory, that momentum and energy would not necessarily be conserved in each interaction but only overall, statistically, was soon shown to be in conflict with experiment.
You could check the Wikipedia article for little more details. Anyway, I don't really understand what it means. I don't think they were saying that the laws of energy conservation and momentum conservation don't hold at all because these two laws, in my view, were very firmly established by then. I think they were saying that energy conservation doesn't hold when you look at individual interactions but it only holds when you look at the complete system. Could you please help me with it? I'm just a learner so please try to keep it simple if you can.
I also find some articles which I cannot access and even if I were able to, I don't think I would be able to understand!