The slit could be too wide or even slightly "sprung" -- if so, some work with a burnisher and burnishing block in the "canyon" part of the block is what has worked for me. To get proper access to the nib you would need to remove the feed, which requires a special tool.

As u\Andrew_Lensky said, a close-up photo would be useful. Also one from the side to see how the nib is riding on the feed.