My first suggestion would be to have her teeth checked - if a cat has discomfort in their mouth lack of self grooming is sometimes the only sign.
If there is nothing wrong with her mouth then I would recommend adding a small amount of oil to her food each day - just a few drops or you'll get a different kind of problem! If you prefer to purchase something specific then I have used Vitapet oil (pet shop) with success to help with 'oldies' constant shedding & I'm currently successfully using 'skin soothe & shine' from Healthy Direct for a cat prone to itchy excema.
Age can also be a factor in the poor appearance of a cat & if this is the case then you will need to supplement her grooming - jugglering has great ideas because it's better to use something the cat likes for a much longer time because you'll get a lot more loose hair out & any sort of massage will help to distribute the oils properly through her coat.. Hope your girl is super shiny soon - x.