Can not log into spark


I am a desktop support technician for a medical clinic adn we use Spark for our nurses to communicate with each other. My problem is that some of our nurses cant login. This happens when they forget their original login password and we have to change it in active directory. I have done everything that I know to fix this problem. (changing active directory password, reinstalling the software, making sure they where a spark user in active directory, disabling and re enabling their active directory account) I dont know what else to do to fix this problem, besides deleting their AD account. Which I wish to reframe from doing. Our network administrator seems to think that its an LDAP problem and the other destop tech seems to think that the domain controller is going to crash soon. I was wondering if anyone out there knew of a fix or could assist.