Hello Edith,
The first thing I would do is remove the carriage from the needle bed, turn it upside down and see which cams move when the central button is pressed in. When both buttons are pushed together there is a black cam in the middle of the carriage which moves. This should not move when the top part of the button is pressed in.
Try cleaning the underside of the carriage, removing all the fluff and oil from under the cams and round the buttons and apply new oil to the parts that move. Perhaps you will be able to free the parts that are sticking together. If this does not work, you might need to remove the cover of the carriage to investigate the mechanism inside.
Hope this helps,
Sue
December 2011