Who is the face of your people?
The first thing to look at when looking at the makeup of your staff, is how as a company are you perceived? What do your comms, social media and leaders say about what it's like to work there. I've worked at many a company where this process has been deemed unnecessary “good and talented people should want to work here”. It is fine in principle to say that but often companies can be viewed as unobtainable, inflexible or simply just not for me, and this can be simply based on what people have read or what they see about the company. If you want a talented work force, sometimes you have to sell to them the benefit of working there. The brand name itself won't automatically get you good people, millennials in particularly, want creative freedom, flexibility and clear promotional paths, and they also want to see people like them that have made it in the business.
So look at your website, is everyone featured white and middle aged, do you have an open social media where you feel like you get an understanding of the people there, or is it corporate and formal? If you are speaking to groups of young people about apprenticeships, do you send people that don't connect with them, or speak to them like teachers or parents rather than on a one to one level? Openly embrace diversity, and talk about the good things that you do around this area, don't leave your BAME and LGBT and disability staff groups as some little secret with a brief note hidden amongst the pages on your website. Shout loud and proud about it. You might think that nobody notices but believe you me, when you see someone like you and you feel that a company will support you for being you, then you are more likely to apply, more likely to stay and more likely to celebrate this with other potential employee.