The texture of kefir depends on many factors, so it's difficult to give a canonical answer. Based on the information you've provided, here are some possible reasons.
I'm assuming you purchased the grains rather than got them from a neighbor, as in the latter case you could simply ask the neighbor for advice. So your grains are quite new. New grains need some recovery time from transportation and some adjustment to a different climate, different milk, etc. Put about 1Tbsp of grains into 1 cup of room temperature milk and leave it for about 24 hours. The next day, filter out the grains and put them in a fresh cup of milk. You may need to do this two or three times before the grains recover fully. You will know they are recovered when they are more than double in size, float to the top, and cause the kefir to start separating into curds and whey. Once this happens, you can increase the amount of milk to two cups per tablespoon.
Even assuming healthy grains, 12 hours is almost certainly too short a ferment. Kefir is at its thickest and creamiest just before it begins to separate into whey and curds. This makes sense: the liquid has thickened to the point where the solids are not yet heavy enough to precipitate, so it is creamy, but once the solids get heavier, they sink and you get separation. So leave your kefir out for longer. Generally 24 hours is the norm, but you will need to experiment. Check at 18 hours. If you see small pockets of whey in the liquid, you are done. If the liquid is more separated, then 18 hours is too long; if you don't see any pockets of whey, leave it for longer. Basically, leave it out for just as long as you need for the texture to be right.
Grain storage. If you are straining out the grains at 8 or 9 am and adding them to milk at 8 or 9 pm, what are you doing with the grains in the meantime? To remain healthy, kefir grains need to be in milk all the time. They cannot survive well by themselves out on the counter or in the refrigerator.
Too much starter. If you have too much starter for the milk, new grains don't get enough sugar to grow and stay small. They then make the kefir gritty. You say 1Tbsp, but not how much milk. Once you are past the recovery stage with your grains, 1Tbsp is right for about 2 cups or a half liter.
Weather and temperature. Dom's super-useful kefir site says:
Kefir can often turn out having a gritty mouth-feel. This may usually occur during mid-season, as the organisms are adapting to change in temperature [the microflora is trying to find its feet]. However, kefir with a gritty mouth-feel is not impaired in regards to health-benefit and nutritional value. Texture and consistency has more to do with the drinking pleasure of the beverage, and the majority of folks seem to prefer kefir with a smooth, creamy consistency.
It's possible that Singapore is too hot for your kefir grains to be comfortable. Using colder milk and a longer ferment might help. Make sure that wherever you leave the kefir to ferment, it is out of direct sunlight. Or of course, perhaps you're leaving the kefir in too cold a spot, but given that it's Singapore, that's pretty unlikely.
Of these possibilities, I think a combo of the second and third is the most likely in your scenario.
Making kefir really is trial and error, though. I too have had gritty batches sometimes. This was a particular problem before I managed to find a process and time-table that works well for me. I now get good results pretty consistently, but it took playing around to get to this point. Here's what I do:
- Use the right amount of starter. I use about 3Tbsp for about 800ml of milk.
- Use cold milk. If I use room temperature milk, my kefir is done quicker than I can drink it.
- Add the milk to the starter and stir well.
- Leave to ferment, covered, in a dark place for roughly 24 hours.
- Stir well again.
- Strain out the grains and immediately start the cycle again.
- The grains keep growing, so every ten days or so, remove excess grains from the strained-out grains and refrigerate in milk.
Hope this helps!