Some additional things to check / test. (Yes, I know some of these are almost TOO obvious, but you'd be surprised how many times the solution is "hiding in plain sight")
Are the campaigns running?
Does the agent have a Dialer license?
Are the agents in the appropriate workgroups?
Is the problem limited to certain agents / machines? Try having agents who can log in from a different machine, use this one. Also try the reverse.
If you install the Scripter.Net client, can they use that?
There are a number of possible places where the issue lies and we need to narrow it down.....