MoM does check the quality of teaching programmes at foreign schools and recognises the ones that are at the same or higher standard as Singaporean degrees. Apparently, yours is of lower quality (or not recognised for other reasons).
Your school could petition against that and provide proof of their good standards for MoM to evaluate again. But this is not an easy or short-term solution.
Please note that it is possible to get a work pass and work in Singapore without a Master degree.