Mine started doing that a lot recently. I accidentally hit the button twice quickly and it worked. Now I just give it two quick clicks and the doors unlock. About 1 out of 10 times, it will unlock and beep because it actually worked with the first click.
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1987 928S4 Silver Metallic (980)/Navy (TP) 5-Speed
2000 Boxster Speed Yellow/Black 5-Speed
1966 Wife White/Brown Top
1986 Daughter White/Brown Top (Sold!)
1992 Daughter White/Blonde Top