Missing the Timing
What is Activation Timing?
Optional “when” effects can only be activated if the most recent thing to happen was their activation condition. If something else happens afterwards, they have incorrect timing.
What can have the wrong Timing?
Any trigger effect which is optional (even cards like Trap Cards), which can be activated “when” something happens, can have the wrong timing.
Any trigger effect which is mandatory (eg, "Sangan"’s effect), or which can be activated “if” something happens, will always have correct timing.
When do things have the wrong timing?
If the last thing to happen wasn’t the activation requirement, that card/effect can’t be activated. The “last thing to happen” can have a variety of causes, but it’s usually because:
The trigger condition happened before a "then" or "after that" effect.
The trigger condition was met at Chain Link 2 or higher.
The trigger condition was met in order to pay a cost.
Example 1: Then
Drill Warrior’s effect reads: “Special Summon this card, then add 1 monster from your GY to your hand”. The “then” means that the last thing to happen is a card being added to the hand, (and not a monster being summoned), so Bottomless Trap Hole can’t be activated.
Example 2: Trigger met at CL2
“Reinforcement of the Army” is activated, and “Call of the Haunted” is chained to it. Chain link 2 (Call) resolves, and then Chain Link 1 (Reinforcement) searches for a card. The last thing to happen is Reinforcement resolving (and not a monster being summoned), so Bottomless Trap Hole can’t be activated.
Example 3 - Costs
“Dupe Frog” is Tributed in order to pay the cost of “Substitoad”’s effect. The last thing to happen was Substitoad’s effect Summoning a Monster, (and not Dupe Frog being sent to the GY), so Dupe Frog’s effect cannot be activated.
Example 4 - Tribute/Fusion/Synchro Summon
“Dupe Frog” is Tributed in order to Tribute Summon “Summoned Skull”. The last thing to happen was “Summoned Skull” being summoned (and not Dupe Frog being sent to the GY), so Dupe Frog’s effect cannot be activated.