Bro, your game is spawning too many zombies, I barely even have time to build plants, and they went through one of them and the plants in the back could've shot the zombies but didn't.
The longer you play, the harder the levels are.
The plants might have a range as a feature for the game to be realistic.
If it's not the answer, it is maybe because you didn't give enough details, but I hope it helps.
The zombies may have been in a different lane to your plants, the 2 furthest away lanes looked like the same lane to me for a while
