As of 30th June 2021:
The three main reasons for an application being refused were due to having a serious criminal conviction, unable to prove identity, unable to provide evidence of UK residency.
If your application is refused, the Home Office will provide you with their reasons and a right to appeal. If the information contained within your refusal letter is not clear, you should contact the Home Office directly to seek clarification. Alternatively, you could seek guidance from a qualified immigration advisor.