A few suggestions:

 - Perform interviews with people who are in a similar role as your users. Try to map out together with them on how a normal workflow looks to them. Who do they interact with? Which parts of the system do they use, and for what purposes? What are their common tasks they usually do?
 - If it's not possible to conduct interviews, find out if there are any additional feedback channels within your company that you can utilize. Have people in-house been part of customer projects? Do you perform training courses with your customers?
 - Create new personas if you identify different users that your original set of personas seem to interact with (i.e. "persona X keeps mentioning this persona Y all the time, and we don't have any data on that... maybe we should look into it?")
 - Spread the information regarding the personas within the organization to further increase everyone's understanding of how the users utilize the system, and what they really value.

And most important of all, validate the personas towards feedback from your existing users, to avoid the personas being based upon speculation.