If your application for a Hong Kong visa is taking a very long time to process any number of things could be going on to cause the delay.
For example, the Immigration Department may be doing deep background checks on you or any of your accompanying family members to assess your bona fides in respect of your identity and your relationship with people included in your application.
They may also be looking into the details of any employer and related overseas entities to better understand their origins and whether or not they could be said to represent a security threat to the HKSAR.
Sometimes, however, the Immigration Department choose not to finalise an application, keeping it open and pending on an indefinite basis as they have concerns about the bona fides and suspect there may be an immigration office being committed by the applicants – but don’t have the jurisdiction to prosecute as it is all happening outside of Hong Kong.
Similarly, if the applicant is in Hong Kong and the Immigration Department have concerns about an application but no evidence of any wrong doing when it basically should be approved all things considered, they may deliberately not finalise an application expecting that prolonged processing time will cause the applicant to withdraw and thereby solving a problem without the department needing to actually come to a decision.
As you can tell, these issues all relate back to the role that the Immigration Department play as part of the Security Branch of the HKSAR, so the bottom line is this.
If it’s taking a very long time, you can be very sure they believe there is a problem.