Ducky and Cera are in my opinion two of the hardest characters to draw. Their bodies are similar to the rest of the gang, but their faces sport all kinds of intricacies which makes them look odd if they are left out.
For Ducky in particular I'd say her nose might be the biggest obstacle. It's kinda weird and in the way, haha. One thing to note is that it should probably run parallel to her mouth. Furthermore, the lighter color of her belly also goes all the way up to her upper lip, which is a small detail I missed for the longest time.
However, I agree with Flathead that using references from the movies is a great idea. Heck, you can even try tracing her for the first time just to get a feel of things, which might make you notice stuff you wouldn't if you just used it as a reference. If you want more specific tips, feel free to share your attempts with us. It's probably better than my first Ducky drawings