I think you are absolutely correct Freddy, pay rates appear to have been stuck for a number of years. When I first came to Thailand around five years ago, the school I worked at paid 30,000 Baht a month. I was told by other teachers that the pay rate had been the same for a number of years. Last time I saw the school advertise on ajarn, which was some time last year, they were still paying 30,000 Baht. So I guess they have been paying the same for about 8 or 9 years.
I think ultimately that schools will only put up pay rate if they cannot get a teacher, and even then only reluctantly. Unfortunately many schools would much rather lower the standard and quality of the teachers they employ than increase the pay rate.