It can take a very long time for them to reply. It took my assessment approx 4 months. I would give them a ring or send a letter asking for an update though.
DWP and ATOS have considerable backlogs at the moment and some members are waiting months to hear anything once they have submitted their ESA50 forms.
As long as you know that the forms arrived on time (which would be highly likely if payments have continued) then you could just wait it out. The only way to be sure of what is happening would be to contact ATOS and ask