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That usually indicates the swtich is defective. You can check this with a small digital multimeter or 12 vDC test light. Just check for good DC voltage OUT of the switch on that side when you try to lower it. The motor works in both directions by reversing the power at the switch. When one side goes bad it only move in one direction. You can replace just the switch, but check the leads to be sure one isn't broken. Hope that helps here. Please get back to me if you need more as all follow ups are included in our fee.
If the switch is bad you need part #736-0009 and most dealers can get this for you from Barker Mfg. Hope that helps here.
Just checking back to see if you were able to test the switch on this? If you need more help please get back to me. Thanks!