We had a nightmare with a bunch of our site servers a while ago. they were working for a while then stopped.
We found that it was related to the dotnet client for the task server.
"2008 Task Servers: Though already mentioned above, there are some items of note fir 2008 R2 severs that run IIS7 by default. The main thing we see is that, by default in version 7.0 and 7.1 sp1, Task Servers often don't work "out of the box". This is almost always a very quick "fix" involving going into IIS and modifying the default app pool to operate in Classic instead of Integrated mode.
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On a server with shared roles, this may not be possible, so you can modify the ClientTaskServer application instead to use the classic-mode app pool. It has the same net effect, and is what we do on the NS.
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To find where to change this, go to Server Manager\Roles\Web Server and highlight "Internet Information Server". Then, in the next pane to the right, expand your server name and highlight Application Pools. Here you can change the Default App Pool (highlight it) and at the right, select Basic Settings. This shows you the integrated option and you can change it to Classic. Alternatively, if you continue browsing in the 2nd pane to Sites\Default Web Site\Altiris, right-click ClientTaskServer, select Manage Application\Advanced Settings and you can switch from using the Default App Pool to the Classic Mode App Pool."
http://www.symantec.com/business/support/index?page=content&id=HOWTO32603
The below help or howto articles were also referenced while trying to remove the problem and may help.
http://www.symantec.com/business/support/index?page=content&id=HOWTO63116&actp=search&viewlocale=en_US&searchid=1333526021757
http://www.symantec.com/business/support/index?page=content&id=HOWTO47803
http://www.symantec.com/business/support/index?page=content&id=TECH47470
http://www.symantec.com/business/support/index?page=content&id=TECH169086
Check your event logs and those on the clients that can't connect to the server and see if anything stands out but the logs are the first place to go after you check the servers are configured correctly.