DTS problem

Hi Guys,
I'm having a problem with DTS, I'll try to explain it as best I can and hopefully someone will be able to help me.
My server is my lappy and client is a Sony tv. Tv is connected to my receiver via optical. If I plug a HDD with movies directly into my tv I have no trouble getting DTS, so it's not the movies or the connection from tv to receiver. When I stream the same movies to my tv with Plex, DTS starts working but only for a second. I've tried changing every setting I can but it makes no difference. I've read heaps of post about DTS not working but if it works for a second surely that means it should keep working? I love Plex but not having DTS is really annoying.
Thanks in advance for any help I can get.
Cheers Mike