I've had the same problem for two years now( 1 of my two vines last year had the problem and my first plant this year is running into same problem) where the baby pumpkin will take and grow for a few days (past the point where an unpollinated pumpkin would have died) and then wither and die once it has attained some size. Its not that the plant is selecting a different pumpkin to send energy to (I'd be ok with that); instead the plant has no pumpkins at all but seems to like killing them off after about a week. I've been giving them a good amount of fertilizer too (start with miracle grow then switch to TNT tomato booster which is a little nitrogren, mostly phosphorous, and some potassium once many flowers have appeared). Any ideas? thank you.