That's one of the lines I was thinking along. So a hearty "+1."
The other "threshold" technique question, was whether the OP was adding the starch already well dissolved in a slurry, or was doing it in some other way.
It seemed like a good idea a couple of hours ago to let the OP give a fuller description of the process before making suggestions, but in retrospect that feels artificial. Whatevvah.
Corn starch isn't what you'd call tricky but there is a right way to do it. It should be thoroughly mixed into liquid and allowed to stand for a couple of minutes, then mixed again before adding to hot, preferably boiling liquid. The slurry should be mixed in completely. As soon as the liquid starts to thicken, the temperature should be reduced.
In the same way corn starch won't reach its thickening potential quickly at less than a boil, it will break down if it's overcooked. So you do want it to boil, but only for a couple of minutes at most -- and it's best added when the liquid to be thickened is already at a boil in order to control the situation.
It's a good idea to make twice as much slurry as you'll think you need, and if the first half doesn't give you the thickening you want after a minute or so, add half the remainder, then if that doesn't do the trick, add the rest.
Cornstarch has roughly twice the thickening power of flour. My thoughts for cobbler are roughly 1-1/2 to 2 tsp cornstarch per cup of "gravy." 1 tbs doesn't seem like way too much, but 2 tbs does -- at least to me.
Another excellent thickener for cooked fruit is tapioca. It takes longer and is trickier to judge -- but you get such a nice gloss.