The bulk approval will only approve leave that has Passed all the rules. One of the rules is that there is enough notice between the request and the leave and when adding some leave retrospectively this rule will fail.
While adding leave retrospectively this rule should be turned off and you can turn it back on by going to Admin > System Functionality > Leave > Absence Rules.
To approve a request that has failed a rule you should click on the Absence Ref on the dashboard to open the approval area and click the approve button. This will return you to the dashboard. You should do this for each leave request in the 'Absence Requests to be approved' box.
It may be that some requests are still not approvable and that will be because of just two reasons. Firstly the leave request may overlap with another request for that employee and secondly the employee might not have enough entitlement to take the leave. The first issue can be checked by going to the employees People page and looking at their leave page. The second issue might be because extra entitlement needs to be given to an employee such as TOIL or carry over or that other leave should have been half days instead of full or booked as a different leave type such as compassionate or appointment.