Agree with Orlando. Most failures to feed are because of a worn op rod spring. Free length of spring should be between 19 1/2 and 20 1/4 inches (per Kuhnhausen). No kinks. Look for worn, flat spots. Clean out the op rod tube. You'd be surprised at the amount of gunk that accumulates over the years. Smear some grease in the palm of your hand and grease the 1st half of the spring that goes down the tube. Tony at Orion 7 sells mil-spec springs at a decent price. As Orlando notes, those "Wolff" springs cause nothing but problems. HTH.
"No man's life, liberty, or property is safe, while Congress is in session." Mark Twain