As Ralph has written, the screws under the rubber strip can corrode (and most likely have to some degree). The external tell-tale is a series of bumps in the rubber as you look at the strip from the top. That can make them a real pain in the #$%^ to get out. When you remove the rubber, assess as best you can what the screws are like. Probably not a bad idea to spray a bit of penetrating oil on them. If you strip the head, then a drill out is probably on the agenda, and that requires care. The drilling process can increase the size of the hole, and then requires a slightly larger screw. Then the head of the screw may not recede fully into the metal strip.
On those you can reach, it is best to try to work them from the back rather than drilling.
Again, as per Ralph, highly recommend replacing them with stainless steel. You will never have to worry about them again.
Also, I would recommend trying to save the rubber inserts if they are the originals. The replacement material comes in a roll you need to cut to fit, and it is difficult to get the rounded edges as the cars had from the factory.