You may have some corrosion around the hinges. If the dogs and the tube are loose, then it should open right up with a push toward the front--or pull up from the deck, on the aluminum frame. I like to use on the teflon lubricants--such as SuperLube spray. This acts as both a lubricant and cleaner. I spray it on the hinges, and into the tubes, as well as taking the knobs off the tubes, and spray into the thread holes.
Remember that the dogs have to completely clear the frame--and the knobs in the tubes completely loose.
Work it very easily--so that you don't break anything--and be sure that each piece is free, as you move it gently.