Accepting task assignments on PPM

Hi,

Whenever we´re assigning a certain task to one or more users, the task appears on the user´s My Assignments tab, however, there is no blue icon clock on the column at the left of the task´s name that would help the user either Accept or Decline the task assignment. When the task line is selected and we click on Accept or Decline, the system shows "1 assignment may have already been accepted/declined".

Delegation of a project to another user, for example, works OK.

Is there any preference/configuration I should be looking into in order to enable this for task assignments??