There are three main reasons why polyurethane parts may have soft spots:

1. The mixing ratio was incorrect

Either too much resin or too much hardener will result in incomplete cure and softer parts

Take care measuring out the components

Ensure the scales are regularly calibrated

2. The components have not been mixed properly

Stir the resin and hardener separately, before stirring in any fillers.

When fully mixed, continue stirring and scraping both the bottom and sides of the container frequently.

If you are not certain it is fully mixed — keep stirring! A couple of minutes longer here can make the difference between a good and a scrapped part.

3. There are excess other components

Fillers and pigments can inhibit the cure; try using less.