Meteor Flower - Firing Rate

I noticed Meteor Flower doesn't performed as per description. Few trials done in Far Future, level 22 (last stand mode). Planted 9 x level 4 Meteor flowers but not all flower firing at 7s even there are still zombie engaging towards Dave's house. 

Only 2 to 3 volleys of meteor instead of 9. Request the team to check for bug fixing. Apologize if I mistakenly understand on the plant description or miscounting. 

Thank you.