Spider Roof is a perpetual problem
I drive a 2007 Spider 2.2 JTS. since new, roof locking is a very common & annoying fault. Alfa Main Dealers have not been able to rectify this problem, & now, 8 years on I have just about had enough. It has now got to the point where I just don't want to put the hood down because invariably the hood will not successfully close. Also, even when kept closed the latches will regularly fail & re-latching again is a laborious task. In my opinion get rid before the car drives you mad, it will get worse, believe me.
You can try is topping up the hydraulic pump fluid.
Also, there are two damper rams attached to each mechanism on each side of the roof. These damper rams work to hold the over-centre levers of the roof mechanism in the closed position. If these damper rams fail to hold the roof mechanism in the closed position they may have to be removed by undoing the clip on each of them. Once removed, take these damper rams to a gas-strut engineer for re-gassing, & then re-install.