Missing the Timing: Because the effect of “Pinch Hopper” effect is an optional “when… you can” Trigger Effect, “Pinch Hopper” being sent to the Graveyard has to be the very last thing that happened in order for you to activate its effect. So if you Tribute “Pinch Hopper” for a Tribute Summon, you cannot activate the effect of “Pinch Hopper” because the last thing to happen will be the Summoning of the Tribute monster. Also, if you Tribute “Pinch Hopper” for “Cannon Soldier”, the very last thing that happened will be the 500 points of damage inflicted to your opponent, so the effect of “Pinch Hopper” will not activate.
If you offer “Pinch Hopper” as a Tribute to attack with “Insect Queen”, you can resolve “Pinch Hopper”’s effect after attack declaration.
When your opponent controls your “Pinch Hopper” and “Pinch Hopper” is sent to the Graveyard, “Pinch Hopper”’s effect does not activate.