They can be touchy sometimes, but once you're on to them it's a good go. Cold molds or plastic leads to incomplete tails. Hot plastic or too much pressure can lead to some flashing in the body vent. Warm molds, 360ish degree plastic and moderate pressure has always worked for me.
Someone else here may have other tips or tricks for you but if all else fails get a hold of me and we'll try to work it out for you.
Thanks,
Jerry