Serious bug

The plugin is nice to work with, but it has a very serious bug. Some of the users who registered with the Contact Form are not displayed in the list of submissions. There are just blanks lines, even though some of the fields in the form I had created are obligatory, so no one can submit a blank form. It happens approximately with 5-7% of all registered users. I really do not know the reason, as I had tested registration process myself with various browsers, and it worked. But I also know that the problem is with the plugin, not the users, since all registrants are independent and registered from different places. So now I have several blank lines, and I simply do not know who they are. I got to know that one of such users had thought that he successfully registered as after registration he was able to read the success message.