Expandable notification not working...help please!!

[meniki2009]

i have samsung gt-s7262 phone. yesterday i did factory reset on my phone. After that i have not been getting expandable notifications which i used to have before factory reset. i am getting collapsed or grouped notifications. for example: earlier for whatsapp, if i had got 5 messages from 5 different people then my notification bar used to show individual notification for each message with a message preview but now it shows as "5 messages from 5 conversations"

please suggest me how to fix this issue....