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