Recently my fiancee bought a Cuisinart ICE-20 Ice Cream Maker. We figured it was a sound purchase because I consume ice cream at least once daily during the summer, and because this particular model got great reviews on Amazon. No ice or salt involved, just freeze the metal canister for 24 hours (it has some magical freezing liquid inside its walls), mix up a batch of fatty dairy products and sugar with your choice of flavoring, and churn for 30 minutes. So easy a monkey could do it.
Except that we couldn't. Our first batch stayed liquid. Our second batch also stayed liquid. Our third batch was a custard base and remained custard (although it was the most delicious rich creamy chocolate custard I've ever had, it wasn't ice cream). At this point we were desperate. The maker itself is so simple that it couldn't possibly be defective. The only explanation was user error. Apparently we aren't as smart as monkeys.
After several trials and more errors, as well as internet blog searching and consultations with my brother, who owns an Ice-25, we think we figured out what was wrong:
1. The canister must be frozen for at least 24 hours. Even if you shake it and don't hear any liquid sloshing around in there, it's not frozen all the way until it's been in the freezer for a day and a night.
2. The liquid ice cream mixture must be as cold as you can get it. If the recipe calls for a custard base or involves heating the liquid in any way, cool it down in the fridge for at least 4 hours.
3. Add extras (chips, nuts, shavings, etc.) at the very very end, when you can already tell that the ice cream is done. Churn just until mix. Then freeze.