Update ComponentNotRerendering.md

docs/FAQ/ComponentNotRerendering.md

@@ -1,15 +1,47 @@
 ### Question
 
-My component is not rerendering, what should I do?
+My component is not rendering, what's wrong?
 
 ### Answer
 
-This could happen for a few reasons. Try the following:
+Ask yourself this question: why do you think the Component should have updated? There's two possible answers: 
+  - a ratom (used by the Component) changed 
+  - the props for (arguments to) the Component changed
 
-1. Make sure you are using a `reagent.core/atom` (i.e., ratom) instead of a normal `atom`.
-2. Make sure to deref your ratom (e.g, @app-state) inside of your component.
+We'll deal with each of these two cases seperately.
+
+### A Ratom Changed
+
+If a ratom changes but your Component doesn't update, then the gotchas to look out for are: 
+1. Make sure you are using a `reagent.core/atom` (i.e. a Reagent ratom) instead of a normal `clojure.core/atom`. Carefully check the `require` at the top of the `ns`.  Components are only reactive with respect to Reagent ratoms. They don't react to changes in a Clojure atom. 
+2. Make sure you actually `deref` your ratom (e.g, @app-state) inside of your component. It is a common mistake for people to forget the leading `@`. 
 3. Make sure your ratom will survive a rerender. Either declare it as a global var, or use a form-2 component. [Read this](https://github.com/reagent-project/reagent-cookbook/tree/master/basics/component-level-state) if you want to understand why.
-4. Make sure to deref your ratom outside of a seq or wrap that seq in a `doall`. See this [related issue](https://github.com/reagent-project/reagent/issues/18).
+4. Make sure to `deref` your ratom outside of a seq or wrap that seq in a `doall`. See this [related issue](https://github.com/reagent-project/reagent/issues/18).
+
+### Props Change
+
+If the props to a Component change, but it doesn't appear to re-render, then the cause will be this rookie mistake: you forgot to repeat the parameters in the inner, anonymous render function.
+
+```
+(defn outer 
+  [a b c]            ;; <--- parameters
+  ;;  ....
+  (fn [a b c]        ;; <--- forgetting to repeat parameters here is the mistake
+    [:div
+      (str a b c)]))
+```
+
+If you forget, the component renderer will stubbornly only ever render the 
+original parameter values, not the updated ones, which can be baffling for 
+a beginner.
+
+Remember, `outer` is called once per component instance. The parameters to `outer` 
+will hold the initial parameter values. The inner renderer on the other hand, 
+will be called by Reagent many times and, each time, potentially with alternative 
+parameter values, but unless you repeat the parameters on the renderer it will 
+close over those initial values in `outer`. As a result, the component renderer 
+will stubbornly only ever render the original parameter values, not the updated ones. 
+
 
 ***