I have loaded a shapefile into QGIS that has been generated in a third party piece of software. When I view the attributes table, all items are there (20 survey lines), when I view the map only 19 appear. I have also tried adding an extra feature in the Attribute table and giving it the same field information as the missing item but this is also not drawn. Any ideas?
First select from the attribute table the feature which is missing and save it as a separated layer. Is it still undisplayed?
Try to check its geometry using the field calculator. Add a new field with
geom_to_wkt( $geometry ) as expression.
(If you have only the one feature you don't even have to add the field, you have a preview after typing the expression)
Won't fix your problem. But now you know its geometry you will be more aware of what's happening.
If every shapefile is missing the last feature then I would think the error is coming from the "blackbox" exporting to shapefile. You should ask to see the code that is used for exporting to shapefile.
The issue feels similar to this one but it is corrected in GDAL four years ago https://trac.osgeo.org/gdal/ticket/5608. The changeset https://trac.osgeo.org/gdal/changeset/27577 does even contain a test case.
Do you know if the blackbox software happens to use an ancient shapelib version which is less than 1.2.7?