What it means to "Miss Timing"

There are 4 different ways of writing an effect that involves timing. Let's take a look at a few examples.

Example 1: Archfiend of Gilfer
Effect in question: When this card is sent from the field to the Graveyard: You can target one monster on your opponent's field; Equip this card to that target as an Equip Card.

Now here, there are two imporant, key phrases to note. Those are "When" and "You can". The combination of these two phrases is what causes the phenomenon known as "Missing Timing". In a "When X happens, you can", if X is not the very last thing to happen in that moment, then the effect misses timing and cannot be activated. For instance, I tribute Archfiend of Gilfer for Caius the Shadow Monarch. Now the last thing to happen is Caius being Normal Summoned, not Gilfer being sent to the grave. The effect misses timing.

Example 2: Sangan
Effect in question: When this card is sent from the field to the Graveyard: Add one monster with 1500 ATK or less from your deck to your hand.

This time, there is no "You can" in the mix. This means that the effect is mandatory. Rather, it is required and must take place. For that reason, the effect can in fact wait for as long as it needs to in order to activate. Effects that are mandatory such as Sangan, Dandylion, and Reborn Tengu will activate when used as part of a Synchro Summon, when tributed, or when destroyed.

Example 3: Eclipse Wyvern
Effect in question: If this card is sent to the Graveyard: You can banish 1 level 7 or higher LIGHt or DARK Dragon-Type monster from your deck.

This time, there's a different wording. It is still a "You can" effect, meaning that it is optional, but it is also an "If" effect. Thing about "If" effects is that they will never, ever miss timing. They can always wait to activate, and are incredibly useful. Even if a monster with an effect like Eclipse Wyvern's is sent to the grave as a cost, it will still be able to activate after the effect has resolved.

To recap, or if just tl;dr:
"When, you can" = Misses timing
"When, you must" = Does not miss timing
"If, you can" = Does not miss timing
"If, you must" Does not miss timing.