The output of this command includes information which can be used to find which activities are blocking the process instances, in particular there are two events for a respective ProcessInstanceID namely TASK and LINK
TASK covers the various types of activity (including groups as well) execution and LINK covers the transitions from the various types of activity (including groups as well) to activity.
To narrow down which ProcessInstanceID to check you may use the "lapi" command
Some of the things from the the output of this "lapis" command are the following:
TASK > done.state is displayed when the activity execution is completed.
TASK > starting.state is displayed for activities which are not yet executed but may yet be executed.
TASK > waiting.state is displayed because the whole activity execution is not yet completed, for example a sleep activity which has begun but not completed will show this state.
TASK > skipped.state is display because the activity was skipped, for example if the transition is with Error condition and the condition was not met so another transition was taken.
LINK > unset.state is displayed where the transition was not yet taken but may be taken at a further point.
LINK > true.state is displayed when the transition is successfully completed.
LINK > false.state is displayed when the transition was not taken, for example when there was an option to take the transition it was not taken due to the condition not met. In addition to this transitions, transitions further down the logic after this transition will also not be taken and they will also have false.state.
If a particular ProcessInstanceID has already been completed then the information for it will not be contained in the output of "lapis" command.
You will see a message containing "Process instance with id [<respective_processinstanceid>] cannot be found."

Please see attached "TestingLapis.zip" which has very simple Project and the output of "lapi" and "lapis" commands done in Studio.