Since this is on a Server, you may want to look at the Firewall settings. Also look at the install logs to see if there are any errors.

if the DLP endpoint installed will have service named "EDPA" & "WDP", you can verify in services.msc.

also check the Article ID: 54753

If the Endpoint agent is not installed or services are not started then the results for netstat will return no results.

If your Endpoint server IP address is, 192.168.2.52  You can perform the following telnet test from a endpoint agent that is not checking in.

open a Command window:

telnet 192.168.2.52 8000

<If the port is open, this command should take you to a blank screen, if it is blocked you will receive a connection refused message>

Another good test you can perform from the Endpoint Agent is a netstat test which will show you what ports are connected/established or listening.
The endpoint server should be listening on port 8000 (0.0.0.0:8000 LISTENING),  The endpoint agent, if connected will show ESTABLISHED on port 8000

Example From Endpoint server:  

<Endpoint Server> C:\>netstat -aon | find "8000"
  TCP    0.0.0.0:8000                0.0.0.0:0                         LISTENING       2192
  TCP    192.168.2.52:8000      192.168.2.53:1433      ESTABLISHED     2192
  TCP    192.168.2.52:8000      192.168.2.54:49306     ESTABLISHED     2192
  TCP    192.168.2.52:8000      192.168.2.55:49160     ESTABLISHED     2192

<The endpoint server example above shows that the server is listening on port 8000, and that 3 Endpoint Agents are ESTABLISHED (192.168.2.53,54,55)>

You can perform the same test from the Endpoint Agent. Here are the type results you "should" see if the agent is connected.

<Endpoint Agent> C:\>netstat -aon | find "8000"
  TCP    192.168.2.53:1433      192.168.2.52:8000      ESTABLISHED     2016

Also try and restart the Monitor Contorller.. and the services on the Endpoint Server too.

is the detection server showing running in the console?