|The person that you described as “most popular” probably meant more to your group than merely scoring high on the popularity scale. What you may be seeing is part of what is called social network theory. The idea is that many groups have one key individual who is the hub or focal point, and this person takes whatever messages are imparted to the group and, by his or her own words and actions, determines whether the message will be absorbed or shelved.
When the person who plays this role in a group is removed for any reason, it is not uncommon to find that the group struggles, especially when it comes to communication and training. Your situation probably is not a case of employee depression or anger at you, but rather a case of floundering. The person who helped them make sense of so much of what they were hearing is gone.
As a result, the message that the employees are sending to you is that they need more guidance, direction, feedback, and follow-up at this point. This means that it will be helpful for you to do more managing by wandering around and increasing the level of two-way communication with each member of your team. Hopefully, one of them will morph into the next natural leader of the group.