Can cornstarch get too old?
Thickening with cornstarch has always been hit or miss for me - today I thickened some fabulous (IMHO) lamb jus (about 1.25 cups) with the properly done slurry of cornstarch (1 tbsp CS/1 tbsp cold water) ...... and nothing material happened.
I served it anyway - it was delicious but I would have liked it to be thicker. Reducing it further wasn't an option because the flavour was perfect.....reducing it would have made it too salty/too acidic.
I make a great roux for gravy but didn't want that effect for the lamb jus.....maybe try arrowroot?