anyone can help pls? 

In the absence of any more details, we can only guess that the feed rollers have developed flat spots (from being in the same position for decades while the rubber hardens) so that when you feed paper the round parts of the rollers will feed it until the flats come into position and then... no more pressure against the platen. Check that and let us know what you find.

@harrysal, Sure. To alleviate the problem, if flat feed rollers is in fact the cause, you replace the feed rollers, in one way or another. The best fix is to have them recovered by J.J. Short, https://www.jjshort.com/typewriter-platen-repair.php . The most effective cheap way is to determine the diameter they should be (possibly the largest remaining dimension) and then remove the flatted rubber and press on a length of rubber hose of the right size (a common source is an automotive supply store). Other fixes involve building up the flat spots with tape or Sugru or such. HTH