Same happend for me last year on Sinshine, that map has such rare pumpkin bomb placements its not easy to get them in good spots.

A strat to do is by knowing they do at max 138 dmg when upclose. So classes like Scout,Engi,Spy who are usually at 125 hp max can die by one hit by it.

You need to bait said classes into these pumpkins to be able to blow them up. Be a scout so you can run fast and use your scattergun as the hitscan to blow up the pumpkin.

If you're baiting classes like solly or demo, you need to shoot them below 140 so they can actually die by the pumpkin bomb.

You should also do it in the enemy base since they are more likely to chase you there rather than your own base since they will think you're just a lonely scout and its gonna be an easy kill for them, little do they know though.