UK Car Repair Questions? Ask a Mechanic for Answers ASAP
There are a couple of latches in there that need to engage in order for the top to lock down Over time they wear and it may take a lot of force to push it down. More so if there is something caught in the roof area. With the roof up, make sure there is nothing in the back area below the window, or along the sides. I have found tennis shoes, scarves, and even a nice pashmina in there.
Next, make sure that the roof inner liner is not loose from the clips along the side. It is a common issue that the cables may break from the eyelets along the rails, and then the inner roof liner gets stuck in the mechanism when closing. As you start to lower the roof, watch to make sure the inner liner "folds" correctly and pulls the inner corners toward the edge.