usually the knob has a threaded metal insert, and a screw goes from the inside of the drawer into the knob.
If yours are like that, try tightening the screw with the correct type and size of driver. First, take off one knob, drive the screw fully home, test it against the thickness of the drawer front to see if it is too long to go tight.
You can get washers to put under the screw head, ebay is a cheap source of a packet of 50. BZP will do. Washers are sold by size in mm which is the diameter of the screw they fit. Yours are probably 4mm. "C type" are extra-wide.
Also buy a small tube of nutlock or thread sealant. A drop in the female thread in the knob (not on the screw) before you do it up will go hard and prevent the screw going loose. It can still be undone with tools. Nail varnish can also be used. Make sure you have got the knobs to fit tightly, and fitted the washers, before using threadlock.
If you have wooden knobs and a woodscrew, you are probably doomed.