SEO efforts only working in one region

Hi SEO community, I'm currently working with a MNC with a global site catering to different regions through "/en-xx" depending on the country it serves.

Back linking efforts were put in place and being based in Singapore, we do see results in SG bringing the website ranking up to rank 1. However, this success is not replicated on the other regional locations that we serve, eg. Philippines or Indonesia.

Why does the SEO efforts not reflect on other regional SERPs?