You don't say how you are sowing to know what might be wrong, and I don't know weather in Malaysia but if it helps at all -

Germination: temperature of 65-75F. Sow with no cover as light aids in germination. Germination may take as long as 30 - 90 days and may be erratic, meaning not all seeds will germinate at the same time.

If you have covered the seeds instead of surface sowing it could cause failure to germinate, and if your weather is much warmer (or colder) than 65-75F it could slow or inhibit germination too.

The temperature in Malaysia is usually about 75F to 86, and can go no lower. If My aunt and grandfather can grow them, then it shouldn't be that much of an issue.

I've also sowed them on the surface as directed, no cover.

How much time are you giving them before you consider they may have failed. If they could take 30-90 days at ideal temps, they could be self sowing at your relatives gardens in more than the typical time frame then, weather being warmer than what might be considered ideal. Again, either warmer, or colder could inhibit or delay germinaton....but that is a guess.

Is it cooler indoors? You might try a trial seed pot inside, gradually move outside soon after germination, increasing their exposure to outside temps and humidity by a little more time each day. Cover the pot with plastic wrap after sowing, check frequently, remove the plastic at first sign of any germination.

I gave them about over a month, since most sites say germination time is about 10 days.

Doesn't it need sunlight to germinate? Or artificial light will do? The design of my house is done in such a way that no direct sunlight gets through the windows. It is cooler in the house if the air cond is on, but it's not always on.

Sometimes the house is warmer than outside, depending on the weather; it's rather erratic during this time of the year.

Perhaps the problem is the seed that you harvested. Maybe it wasn't ripe, or isn't viable for some other reason. Good seed should germinate quickly.