I used agar-agar as a substitute for gelatin on a strawberry filling for a cake and it turned out very runny. (Customer is vegetarian and ask me not to use gelatin) What can I do to make it thicker?

I have never used agar before, I found out in the internet that you can substitute one tablespoon powdered gelatin for every tablespoon of powdered agar, and that is what I did.

I'm vegetarian also and have used agar on occasion. I do remember it's not a one to one substitution, less agar than the measurement of gelatin) and you need to boil the agar, unlike gelatin.

Or you can thicken with cornstarch.

Sorry, don't know much about agar.