Someone asked you if you had tested it? My unit was doing the same and after reading the literature on the unit I hit the test button. It roared to life and the problem went away. I had read somewhere the dust can settle on the detector rendering it problematic. The test is supposed to remove said dust. Mine says to test yearly. I see where yours says weekly. Have you done this?
Checking the power supply is the first thing. If that doesn't help, try doing the test. Mine is a CO tester. Haven't had any issues with the propane tester.