--"Can't recycle X item?"
As of v6.1.0 of this mod, you can no longer recycle intermediate products using the lower tiered machines. Only the t4 reverse factory can recycle intermediates.
If it's a vehicle or power armor, then you have to enable that setting (which is off by default, to protect those items' internal inventories and equipments, which are lost upon recycling).
If the item crafts in pairs (such as yellow belt) or greater, then you will need that same amount of material to recycle back into its constituent parts (EX: 2 yellow belt back into iron plate and gears)
If it's a dense recipe, like the nuclear reactor or rocket silo, then you will have to disable the safety setting (which is on by default, to prevent the unintended deletion of resources). See "dense recipes?" section for further information.
[If it's some other item that doesn't fall into these categories, then it's probably a bug, and you should post about it in the discussion tab. Be sure to include any "big" mods or potential mods which might affect the item in question, especially if the item is from a mod itself.]

--Note: The safety ingredient toggle has been removed as of v6.0.7, as Factorio logic has changed to allow multiple item stacks to fit in a single item slot, which was not previously possible. No resources are lost, and items such as the rocket silo, nuclear reactor, etc can now be recycled without issue.
--"Dense recipes?"
I use the term "dense recipes" for recipes whose ingredient counts are larger than their stack sizes. An example of this is the rocket silo, which requires 1000 concrete in vanilla, but concrete only stacks up to 200 in vanilla. Due to certain engine changes introduced in 0.14 or 0.15, recycling a dense recipe produces only up to the stack limit, meaning that 800 excess concrete would be lost.
In vanilla, the rocket silo, nuclear reactor, satellite, and power armor mk2, are among the few dense recipes.
If you wish to recycle these items, then you must disable the ingredient safety toggle, located under Settings > Mod Options

--"Duplication bug?"
If it involves a recycler and an assembler with production modules in it, looping between each other, then I know about the issue, and I don't intend to fix it. Anyone who wants to exploit this mechanic for infinite resources, go ahead! I don't personally care for it, but anyone else should feel free to play as they wish.
If an item is producing more items than it took to initially make, that is an unintended bug, and should definitely be reported. Again, include any relevant mods, especially if the item in question is itself from a mod. This is most likely to occur when using two mods which add similar items/recipes.

--"Defining custom recycling recipes?" (for mod creators)
If the items in your mod are not automatically added to the list of recycling recipes (due to item name and recipe name not matching), then you may add entries to the custom recipe list, by defining the item type, item name, and recipe name in data-updates via:

table.insert(rf.custom_recycle, {"item", "example-item", "example-recipe"})

--"Setting unrecycleable recipes/categories?" (for mod creators)
If you would like to define a set of recipes or categories from your mod that should not be recycled (either for balance purposes, or to improve compatibility/prevent errors on load), then in data-updates, you can add to this list via:

table.insert(rf.norecycle_items, "example-recipe")
table.insert(rf.norecycle_categories, "example-category")

Always be sure to preface these lines with a check for this mod, using something like:

if (rf) then
  if (rf.norecycle_items) then
      --logic goes here
  end
end

Please note that recipes whose names do not match the item(s) they produce will automatically be ignored, and do not have to be defined. Also please refrain from adding vanilla categories to this list.