Ukrainian last names like Kulik, or English ones like Davis do not add an -a, and neither do Russian ones like Ilinykh. To add an -a, a name must end in -ov/ev or -in. It doesn't have to be Slavic, other ethnic groups like Tatars also have last names ending in -ov/ev and -in which add an -a (Valieva, Zagitova, Khudaiberdieva, Mustafina etc).StepanovA
SmilkovA
Russian women usually add A to their last name. The only exceptions I have seen are Liza Kulik, Diana Davis and Mirozov's daughter. All three were born in the USA. There could be others though, and born in either Russia or Ukraine, with names ending in O. The Slavic names and those ending 'in' do add A.
Examples - Boikova, Moskvina.