If you overcook cornstarch it will break down and go watery. I'd cook it just until it was thick, then stop. Residual heat will keep it cooking so even if it seemed right when you poured it out before, the residual heat could have taken it past the overcooked point.

Choose another thickener: arrowroot, kudzoo, potato starch, Clear Jel - even all-purpose flour is sometimes used. You'll find more information at the link below.

-Grainlady

Here is a link that might be useful: Baking 911 - Pies & Tarts

Corn starch is fine....but it's touchy. As colleen says it will break down if you over cook it.
As with all recipes and cooking you have to go by "cooking until it's done" rather then time as it may vary, and "allow to rise until double" rather than raise for 1 hour.
Try it again, if you have the courage....and cook until it just "breaks a boil"....and turn the heat low so it just blurps like one of those mud pots in Yellowstone park, and cook until just thickened.....about 3 to 5 minutes after it first reaches the boiling point.
Or try arrowroot....but I find that can sometimes get "ropey"...unplesant texture, if you can get it, clear jel is best for cold fruit pies.
Linda C

As I think grainlady has mentioned before, a lot of recipes are flawed. In this case the instructions are poor. Stirring too vigorously can cause cornstarch to break down, as can boiling. It needs a gentle hand.

Also, it may not be your fault. It may be the strawberries. If they're more acidic, the cornstarch glaze may be fine initially but won't hold over time.

ClearJel is by far the more reliable product. Usually you have to order it online but I have found it in cake decorating stores. There's an instant form and a cooked form, which could be used with this recipe.

Carol

That's almost exactly like the recipe I use, except I use exactly 1 cup of sugar and add 1 tablespoon of fresh lemon juice to the cooled berry mixture. Last week, I got a little carried away with the lemon reamer and probably put more than 1 tablespoon of juice in it. It set up kind of runny, too. Never thought of using ClearJel. Would you sub the same amount?

That's awfully close to the recipe I use and it always sets up. Don't be put off. Something weird happened (that's the scientific explanation) try again using Coleen's caution. It's too good and easy a recipe to abandon....Tache

If you use ClearJel in lieu of cornstarch, you can use it 1:1, but I back off a little. I find it too viscous for my taste.

Carol

I always use tapioca and it works great. I make sure to buy a fresh box each year though.

Okay, I've lived in the South for almost 40 years and the only kudzoo I know is that awful stuff that drapes over trees and kills them. I know that when it was originally brought to this county it was supposed to be used for cattle fodder and Southerners have been sorry ever since!

Can this be the same thing? Some good purpose for this awful plant? Tell me more!

Cook's Thesaurus mentions kudzu as a thickener. I think I'll stick with cornstarch, ClearJel or tapioca.

I see they describe kudzu as an "obnoxious vine." True, true, true.

Carol

Here is a link that might be useful: Cook's Thesaurus: Thickeners

If you taste the sauce as it is cooking and then, without washing off the spoon,stir the sauce again, the enzymes in the saliva with cause the cornstarch to thin.
I know probably no one does that but it was something I learned in chemistry when I was in college.

Oh yes,Carol, my chemistry class taught the same thing and it is particularly important if you are cooking chocolate. In fact you need to taste it several times.

To the OP; I dug out my old recipe and I think you might want to try yours with 4 tbles. cornstarch. I'm not a great cook and it has never failed for me. It has such a wonderful fresh strawberry taste

If the cornstarch breaks down overtime with heat and stirring, cant you heat to thicken (isn't this basically evaporation?) and add the cornstarch a little later?

No it won't "re thicken"....essentially the starch molocule that forms a network to hold liquid and make it "thick" breaks down.
I can't remember the exact chemestry but that's close.
Linda C

Thanks, everyone. I'll go buy some strawberries and give it another go, this time increasing the cornstarch a little and not cooking it too long. If that doesn't work, I'll try some of the other thickers.