Try lubricating all of the joints in the top. Mine would not get started without a "push" when trying to lower the top.

I opened it up to a point where I could get to all of the rivets that hold it together, and put some light sewing machine oil at each pivot point. Raised and lowered it a couple of times and it works fine now.

I think the motor has only so much power. over time the joints get tight or get dust in them. Easy enough to try.

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2008 Space Gray Z4 3.0 si, Saddle Leather interior
2011 Nissan Frontier King Cab
2010 Lexus RX350