Missing the Timing: “Peten the Dark Clown” is a “when… you can” optional Trigger Effect. This means that “Peten” being sent to the Graveyard has to be the very last thing that happened in order to activate its effect.
If “Peten the Dark Clown” is Tributed for a Tribute Summon, the last thing that happened was that a monster was Summoned, so the effect of “Peten” does not activate.
If “Peten the Dark Clown” is Tributed for “Mystik Wok” or “Cannon Soldier”, the last thing that happened was that those effects resolved, so the effect of “Peten” does not activate.
If “Peten the Dark Clown” is destroyed in a chain and not as Chain Link 1, his effect does not activate. (Example: “Pot of Greed” is activated and “Ring of Destruction” is chained, targeting “Peten”.)
If “Peten the Dark Clown” is destroyed by “Cyber Jar”, the last thing that happened was that monsters were Summoned to the field, so the effect of “Peten” does not activate.
If “Peten the Dark Clown” is discarded as a cost for “Tribe-Infecting Virus” or “Raigeki Break”, the last thing that happened was that those effects resolved, so the effect of “Peten” does not activate.
If “Peten the Dark Clown” is discarded because of “Morphing Jar” or “Card Destruction”, the last thing that happened was that both players drew cards. So the effect of “Peten” does not activate.
If “Peten the Dark Clown” is sent to the Graveyard as a result of battle, or by “Painful Choice”, the last thing that happened WAS that “Peten” was sent to the Graveyard, so his effect will activate.