I don't know, but I had the same problem making skulls, now they look like shrunken heads lol

 

One part polyurethane foam (Great Stuff, etc. ) needs a little moisture to cure. If you apply too thick of an amount it doesn't get enough atmospheric moisture. This results in gooey centers and shriveling/shrinking.

To prevent this, mist the area you want to foam, apply a single layer of foam, then mist lightly with water again. Lay down another layer, mist again. Continue until the desired amount of foam has been used.

 

LIS that is exactly what I did, and I still have the shrunken head look,IDK maybe it was making them in the cold basement them bringing them outside in the heat to finish cure.

 

Huh. Maybe it was the temp. change. I've only had temp. problems when I stored "not quite cured" foam in the shed, in the summer. (Bubble collapse) Yeah, not the best place for latex and polyurethane... I know!

The only thing I can think of is perhaps try a finer mist of water. Just guessing here though!

Lemme know if you find out the problem.