It is quite common for gedcom files to contain errors, and some errors may cause GWintree to be unable to produce a chart.
Gedcom files may also vary a good deal in the way that records are numbered and ordered within the file, which might be the cause of a problem. To eliminate this possibility, choose Renumber Records from the Tools menu, and afterwards save the file under a new name. This will cause the program to scan and fix errors.
You can also fix errors in a gedcom file, after first loading the file into the program, by opening view/edit mode, clicking Save on the toolbar, and saving the file under a different name. If you then load the new file into the program, you should be able to produce a chart.
When you save a file in GWintree, the program checks the data and fixes some types of errors. Checks are made to ensure that the cross-references between individuals and families are consistent, and that husbands and wives are of the correct sex. If, for instance, a woman appears as the husband in a family and the man as the wife, the references will be swapped. However, if the spouses are correct, except that one has been assigned the wrong sex, that person will be removed from the family. After saving a file to fix errors, you should check for unlinked individuals by clicking the radiobutton to display the list in view/edit mode.
Choosing Preferences... from the Options menu and selecting Scan and fix errors on loading Gedcom file will cause the program to scan and fix errors every time a gedcom file is loaded. The time taken to load a large file may be significantly increased, however.