Sophie,
A task being Active is not the same as Executable. Please add the Executable column at the Work Plan and verify that the work item is executable (green icon). Also, how is the time reported on the work item? Could it be possible that you have multiple users and the leaf reporting is set to Individual instead of shared...and this user has entered there full time allocated? This would lock the user from adding more time as their portion of the time entry is complete.
Further troubleshooting may require you to open a ticket and provide us access to review.
Sincerely,
Boris