We have a Cuisinart frozen yogurt / ice cream maker. It's the kind
where you leave the container in the freezer, then when you're ready
to use it, you load the ingredients into the container and place it on
the motorized base for 30 minutes.

We made frozen yogurt using fresh blueberries the other night.

We put it into the freezer for a couple of hours, and it froze solid
as a rock!

Last night we took it out and figured we would let it sit for 10
minutes or so. Well, after an hour it was still rock solid! You
couldn't even dig into it with a spoon.

So this morning, my wife took it out after breakfast. Six hours
later, we can barely get a few spoonfuls off of the top.

Is there something that could have been added to keep it soft in the
freezer? Or should we just have eaten it all at the time it was made?