I've lost my 5.1 sounds on SD channels
As a general rule, SD channels do not broadcast DD (5.1 or otherwise). The notable exceptions are the Sky movie channels ... but they are (completely ?) replicated in HD, so you should be watching the HD variants anyway (assuming that you have an HD subscription of course !).
If you tell us which programmes/channels you think have been affected then I'm sure others here will be able to elaborate.
The amp itself auto detects Dolby D, and used to auto detect PLII...
Most Home Cinema AV Amps will indeed detect DD and give you up to six channels of sound (i.e. 5.1). However, I don't believe that Dolby PL (I or II) is "detectable".
Dolby Pro Logic (unlike Dolby Digital) is not a transport
format comprising multiple channels ... it is a way of encoding surround sound info for extra channels within a
normal two-channel stereo signal.
In other words, your amp doesn't "know" whether a stereo signal contains extra DPL info any more than it "knows" that the music contains a saxophone (or any other specific content).
What you
can do is set the amp to perform a DPL decoding procedure on an incoming stereo signal. This will create a couple of extra channels; a Front Centre and a Rear channel. The FC channel is sent to your centre speaker and the R channel is sent, equally, to your two rear speakers. The downside is that these extra channels are relatively poor quality (specifically of limited frequency range) and there is no stereo effect to the sound behind you. Some amps also route the lower frequencies to your sub-woofer, but all this does is help out systems with relatively poor speakers - there is no extra sub-bass information to really warrant the use of a sub-woofer.
I can force it to PLII mode, but it just sounds weird...
If the incoming stereo signal contains no surround info, then the result can be pretty ugly. If you put your head close to the FL or FR speakers you may hear a sort of quiet gurgling sound. Most of the sound can end up coming from the centre speaker. Not only will it now be frequency limited but most centre speakers are not as good quality as the fronts (certainly not if you have added a relatively inexpensive centre speaker to a good stereo set up).
I'm not a "golden ears" kind of guy, but even I have noticed how much the sound can improve by going back to stereo for SD TV. It sounds less muffled and has a better stereo image.
So remember, you aren't
detecting DPL, you are choosing to
apply a DPL decode to a stereo signal. FWIW, you can equally well apply DPLI, DPLII or one of the "Neo:" algorithms from DTS.
It sounds like your default option of DPL has somehow fallen back to stereo. Without knowing more about your amp we can't help you change that (although I'm confused here, because usually amps tend to stick with the last option you selected).
Regards