So sounds like you have existing customers who have signed up for (and are paying for) your software, but not using it as actively as you'd like? Is this a SaaS application (monthly/annual subscription price)?
You're right that for a SaaS application it's really important to keep your users engaged, so they don't unsubscribe. I've been doing software marketing for almost 20 years, so I'll give you a few suggestions. You might also find some helpful ideas at www.software-marketing-advisor.com
First, you really should have a regular (weekly) newsletter for existing customers. Provide them with tips about using your software, helpful ways to manage CRM, etc. Some of the info can relate directly to your software, and some more general tips, but the important thing is to have a call-to-action in every newsletter that brings them back to doing something in your application. Each newsletter could cover tips around a specific topic.
You can also set up action-driven automated emails. So, for example, if someone hasn't logged in to use the software in a specified amount of time, you can send them an email with some extra tips and suggestions ("did you know how easy it is to do XYZ in our software?", "how long has it been since you reached out to your prospect pipeline?" etc)
Hope that helps... send me a private message if you'd like to chat further.