A Swamp cooler needs an occasional once-over. The biggest indicator is if the swamp cooler is not blowing cool air (assuming it’s not really humid outside). If the cooler is blowing air but it isn’t cold, that would suggest that the pads are not getting wet.
Remove a panel and take a look inside the cooler. Turn the pump on and make sure the spider is delivering water to the top of the pads. If the individual arms of the spider are working except one, you know the problem is isolated to that arm.
Scale will sometimes get caught in the arms at a bend in the arm. You can probably dislodge it with a flexible pipe cleaner. Take your finger and first pick off any dried deposits on the end of the arm and then stick the pipe cleaner up there. You can do this while the water is running and hopefully when you dislodge the debris, the water will push it out of the arm.
If none of the arms are flowing water, check to see that the pump is working. If the pump works, you might consider removing the spider at the top of the cooler and investigating whether it is blocked. The spider is held on by a screw on top of the frame. You can disconnect the hose that runs from the pump to the spider and turn the water on. Water should shoot out of the hose. If water doesn’t come out of the hose, the problem is with the pump. If water does come out of the hose, the spider is plugged up and needs to be cleared out or just replaced.