How much should a poolish increase in size?
A few things that could be going on:


0.2 grams of instant yeast is a pretty small amount.  Even a little variation in measurement could make a difference in how fast it rises.  Also, whether this small amount was well-distributed throughout the flour at the outset could affect the timing of the rise.
Is the yeast fresh/good?  Older yeast can be sluggish.
How are you measuring whether it "tripled" in volume?  Many people put dough in bowls that get wider toward the top, so it can be difficult to measure exactly when something "triples" in volume.  Unless you're using a straight-sided beaker or something, or have measurement marks on your bowl, it's possible that something "tripled" in volume while not rising to three times the height.
Have you monitored the poolish?  Did it simply stop rising at some point, or was it still rising at 12 hours?  With high-rising preferments, it is also possible that a poolish might rise higher and then fall after some point, collapsing under its own weight.  If you weren't monitoring this, you might see evidence like streaks higher on the side of the bowl if this happened.
If it simply stopped rising at some point, the question of protein content in the flour may be useful to think about.  If the flour doesn't have enough protein, it won't hold up a very high structure in a wet preferment.  Eventually the bubbles will get large enough that they will simply rise to the top and burst, rather than staying within the dough structure.  It's also important to note that higher protein flours can sometimes absorb water better, so if the recipe was designed for "bread flour" or flour with a higher protein content, the moisture level in your poolish could be high enough that it is producing large bursting bubbles rather than holding the air in the dough.


In any case, if the preferment at least doubled in size and/or is very bubbly, I doubt there's anything truly "wrong" and you should go ahead and bake the bread, which will likely turn out fine.