Read this article.
The comments posted in the website suggest that the problem has been around for a long time, and that the Ministry and its officials have not been doing their job well.
I suspect that the problem is more deep rooted. We are allowing many unskilled workers to work in Singapore at low wages (and high levies paid to the government). Being unskilled, they do not have the language competence and the ability to learn the skills and knowledge.
The providers are probably under pressure to pass a quota of trainees, so they relaxed on their standards. Basically, the root cause is the type of people being allowed to work in the construction industry in Singapore.
It takes years to acquire the skills. It cannot be achieved with a crash course. I suspect that this is the root cause of the problem.
Unfortunately, this is the consequence of the contracting and tendering system that has been adopted for decades in Singapore and the reliance on low cost foreign workers. It does not give the prospects for local construction workers to stay long in the industry to develop the skills.