Users logouts from time to time
Is there a better way to control user's login sessions? Yes.
There's a great module (beta since 2012, 1,000+ active sites used it as of October 1, 2014): https://www.drupal.org/project/persistent_login
From the module page:

The Persistent Login module provides the familiar &quot;Remember Me&quot; option in the user login
form. Additional security is provided by using two tokens (one long-running, one per
access) that allow the detection of unauthorized token use.
Persistent Login is independent of the PHP session settings and is more secure (and
user-friendly) than simply setting a long PHP session lifetime. For a detailed
discussion of the design and security of Persistent Login, see Improved Persistent Login
Cookie Best Practice.

What is causing the users to be disconnected? Depends.
As Mołot pointed out, there are literally dozens of things that could be happening. For example, we could be talking about multiple browsers, multiple devices, or someone who regularly clears out their cookies and internet history. Most of these situations are out of your control. The standard Drupal session cookie usually keeps me logged in over the course of a day or two without having to re-login. If someone is getting logged off in the same 24 hours span, something is happening.