STAT of Process

[export@hope.cz]

I have a daemon running on my Debian ( Lenny) box.When it is  started ,

 I check the process state ( by ps -ax command) and I can see in STAT column  S letter and the daemon works as it should.After some time, the STAT is changed to R but the daemon still works OK.But after  few hours the STAT is changed to D letter and the daemon does not work properly.It seems as if it is stopped running eventhough it is among the  list of processes.

Can anyone explain that please?

Thanks

L.