if ya have some time to test for me set it to not use the sprinkler and hook up your waterlines using the splitters like the planter does and see if there still arises an issue. i sprinkler and one splitter per planter.. I also have noticed if the last planter only had 2 water power then it cant keep up with the plants usage.
tested and seems to be a rust related issue where if sprinklers do not have enuff water they will run out cause they can not keep up So my next question to verify this is if you kill all the plants in the planter does the planter start to build back up water?