Stereo speakers

Are the ones that don't work in a slide out room?
If so, possibly the in and out movement of the side has separated the speaker feed wires...climb underneath the slide and check---if you can.

Are you sure the non-working speakers are Stereo? My Home Theater is a 5.1 surround system (Total of six speakers if you know where to look)

Front center just me side of the TV
Two more right and left a bit closer to me on either side

Two "Rear channel" (right and left) behind me

and a sub woofer hidden well (I do know where it is)

If you have "Stereo classic" profile in the amplifer the "Rear" speakers do not work.

Re-setting to simulate surround (or to surround if the program qualifies) they work.

Typical mobile players detach from a cage by inserting a removal tool through the faceplate. Check with your mfg to get detailed instructions.

To diagnose the problem, you would need to know if there is any signal voltage from the player to the speaker location. That signal will be low AC voltage. To trouble shoot the condition of the speakers you could measure the resistance across the unconnected speaker terminals, it should be in a range around 4 ohms. To identify if the signal is missing from that single location, you could jumper leads from the existing, functioning channels to the non-functioning speaker locations.

Make sure you don't have a button or menu that turns on and off a channel. It could be that simple.