The other day I was troubleshooting a swamp cooler because there were areas of the pads that were not getting wet. Then I pulled the sides off of the cooler and could see that water was intermittently coming from each tube from the spider at the top of the cooler.
At the bottom of the cooler is the pump, and it was much louder than normal. The pump sits in a basket that helps to keep debris from entering the pump and consequently the hose and spider. The drip tubes above the pads appeared to be taking turns flowing water onto the pads. There obviously wasn’t an obstruction, rather the pump was no longer pushing the volume of water up to the spider to distribute to the pads.
I disconnected the bracket that holds the pump to the housing and pulled up the pump to look at it. It was very crusty like something you would see from an old boat that had spent its entire life just sitting in water, less the barnacles of course.
Pumps are cheap, and so I bought a new one and installed it. It flowed plenty of water up to the spider and all drip tubes were flowing a consistent amount of water on top of the pads.