Open the face plate, check the presser foot tension dial. It should be set at 4 or 5 for normal sewing.
Then oil everything that moves as you work it. The presser foot lever will be harder to move if it needs oil or the shaft tension is too tight. There is a lot of things in there to oil. Especially the horizontal shaft that guides the swing arm for ZZ and pattern sewing.
For the cabinet leaves
Remove the screws from the leaf(s) and the cabinet body. Clean the holes.
Make sure the hinges are not bent. If so straighten them.
Then roll up some thin veneer or if you don't have any, match sticks and fill the screw holes mostly up. Before you put the veneer or match sticks in fill the hole with wood glue. After filling the hole slice off the filler wood at the surface.
After the hole is mostly filled and before the glue is dry reassemble the hinges to the cabinet body and leaf(s) and tighten the screws down tight.
I've done this to several cabinets and so far it's leveled up the leafs on all of them.
Joe