I get toward the end of the questline and the final one is survive 2 massive zombie attacks, problem is that the things you need to pick up to get energy so you can put down sunflowers, plants, etc. Problem is they take so long to drop, you end up failing the quest because you can’t collect enough to drop more than 1 plant. Not sure why this is so hard, I remember when I did it back in the day when the questline was first released, it was easy to collect the sun drops and get enough to keep up with placing what was needed to stop the zombie’s, now it is horribly broken.
Are you planting a lot of sunflowers at the start? I think I had 2 full columns of sunflowers going in order to have enough sun power to beat it.