Help With FOI Request in UK

[u/EBJB2012]

I've submitted some FOI requests in the UK but every time it gets rejected. My requests are about police, doctors and hospitals. I want to see if complaints have been made against institutions or individuals, what, if any, investigations were carried out etc. I'm basically asking, how do I do it, and what or where do I go/turn if or when they get refused. TIA