You need to wait longer. Two weeks is too short. Be patient for up to a month for the secondary fermentation to finish. (by "secondary" I mean the fermentation to add carbonation)
Also significantly, 10* Celsius, which is only about 50*F is not sufficient for any portion of your secondary fermentation with most yeasts ... in fact, for any fermentation at all with many yeasts. Even for the common ones which are lower temp such as EC-1118, D47, Premier Cuvee etc.
You need to raise the temp to about 70*F which is about 21*C.
In general, the amount of carbonation you end up with is dependent on a couple things ... the amount of residual carbonation that your cider had from the primary fermentation ... and the amount of priming sugar you put in to do the “carbonating fermentation”.
Though the ½ teaspoon amount of sugar you added could be on the light side ... the proof will be in the pudding.
So ... raise the temp .... wait for 3 to 4 weeks and see what you get and whether you like it.
Lets say you don’t get the carbonation you want ... how much sugar should have been added?
Say you want a decent amount of carbonation similar to a commercial lager .... or about “2.25 volumes” of carbonation. Lets guess that your primary ferment at 20* Celsius left .88 volumes in the cider to begin with and you did not degass too much from handling. This would mean you want an additional 1.3 volumes of carbonation to get it to 2.25.
The amount of sugar you need to add to get an extra 1.3 volumes of CO2 in a 12oz bottle is about 1/2 teaspoon.
But ya never know. Measuring tiny amounts of sugar into small bottles of cider at an unknown residual sweetness and with an uncertain amount of initial carbonation (from the first ferment) to begin with is a bit of a crap shoot.
Maybe after a month the carbonation will be enough that you like it! ... but if it was me, I’d pop the caps, add an additional 3/4 of a teaspoon sugar to each bottle (to be safe) ... re-cap, give it a brief shake and let ‘em sit at 21* Celsius / 70*F.
Open one bottle in two weeks ... another one at 3 weeks ... another at 4 weeks. If at some point you find that you have all the carbonation you want in it ... put the whole batch in the refrigerator to stop the fermentation.
BTW all this fraction of a teaspoon stuff is why priming is best done either in bulk or, if in the bottle, with priming tabs as was mentioned.
For what it’s worth, if you want to read more technical blibber on how to prime for carbonation ... here are a few posts from a while back ... http://www.homebrewtalk.com/f32/cloudy-cider-269568/index2.html
Finally ... you say you put the bottles on a heating pad for 4 days. If you mean like an “electric blanket” that would be ok ... but if it is one of the smaller therapeutic heating pads, there is some possibility of killing some or all of yeast in that 4 day amount of time. Heating pads range from 110* to 160* with an Underwriters Labs maximum of 176* Fahrenheit ... whereas “electric blankets” usually get from about 80* to 120*.
Time will tell if this was the case.